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Filed Pursuant to Rule 424(b)(5)
Registration No. 333-089970

Prospectus Supplement to Prospectus dated August 14, 2002.

19,063,181 Shares

LOGO

Common Stock

        Our common stock is listed on the New York Stock Exchange under the symbol "AEE." The last reported sale price of our common stock on February 3, 2004 was $47.57 per share.

        See "Risk Factors" beginning on page S-8 of this prospectus supplement to read about certain factors you should consider before buying shares of the common stock.

        Neither the Securities and Exchange Commission nor any other federal or state regulatory body has approved or disapproved of these securities or passed upon the accuracy or adequacy of this prospectus supplement or the accompanying prospectus. Any representation to the contrary is a criminal offense.

 
   Per Share
   Total
Initial price to public(1)   $ 45.90   $ 875,000,008
Underwriting discount   $ 1.15   $ 21,922,658
Proceeds, before expenses, to Ameren   $ 44.75   $ 853,077,350

        Goldman, Sachs & Co. expects to deliver the shares against payment in New York, New York on February 6, 2004.

Goldman, Sachs & Co.

Prospectus Supplement dated February 3, 2004.


PROSPECTUS SUPPLEMENT SUMMARY

        This summary highlights certain information contained elsewhere, or incorporated by reference, in this prospectus supplement and the accompanying prospectus. As a result, it does not contain all of the information that you should consider before investing in our common stock. You should read the entire prospectus supplement, including the accompanying prospectus and the documents incorporated by reference, which are described under "Where You Can Find More Information" in the accompanying prospectus. This prospectus supplement and the accompanying prospectus contain or incorporate forward-looking statements. Forward-looking statements should be read with the cautionary statements and important factors included in the accompanying prospectus under "Forward-Looking Statements."

Ameren Corporation

        Ameren is a public utility holding company registered under the Public Utility Holding Company Act of 1935 and does not own any significant assets other than the common stock of its subsidiaries. We were incorporated in Missouri on August 7, 1995. On December 31, 1997, following the receipt of all required approvals, CIPSCO Incorporated, or CIPSCO, and Union Electric Company, or AmerenUE, combined with the result that the common shareholders of CIPSCO and AmerenUE became the common shareholders of Ameren, and Ameren became the owner of 100% of the common stock of AmerenUE and CIPSCO's subsidiaries, Central Illinois Public Service Company, or AmerenCIPS, and CIPSCO Investment Company. On January 31, 2003 and February 4, 2003, respectively, we completed our acquisitions from The AES Corporation of all of the outstanding common stock of CILCORP Inc. and the membership interests of AES Medina Valley Cogen (No. 4), LLC. CILCORP is the parent company of Peoria, Illinois-based Central Illinois Light Company, which we now operate as AmerenCILCO. AES Medina Valley Cogen (No. 4), LLC has been renamed AmerenEnergy Medina Valley Cogen (No. 4), LLC, or Medina Valley.

        Our primary subsidiaries are as follows:

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        Through AmerenUE, we own the Callaway nuclear plant, which represents approximately 14% of AmerenUE's generation capability. On the morning of February 3, 2004, the Callaway nuclear plant automatically tripped off the grid due to a generator relay malfunction. This incident did not relate to any nuclear activity at the plant. We are currently in the process of addressing this problem and anticipate resuming operations in the near future.

        Our principal executive offices are located at 1901 Chouteau Avenue, St. Louis, Missouri 63103 and our telephone number is (314) 621-3222.

        In this prospectus supplement and the accompanying prospectus, "Ameren," "we," "us" and "our" refer to Ameren Corporation and, unless the context otherwise indicates, do not include our subsidiaries.

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Proposed Acquisition of Illinois Power

        On February 2, 2004, we entered into an agreement with Dynegy Inc. to purchase the stock of Decatur, Illinois-based Illinois Power Company, or Illinois Power, and Dynegy's 20% ownership interest in EEI. Illinois Power operates a rate-regulated electric and natural gas transmission and distribution business serving 590,000 electric and 415,000 gas customers in areas contiguous to our existing Illinois utility service territories. The total transaction value is approximately $2.3 billion, including the assumption of approximately $1.8 billion of Illinois Power debt and preferred stock. Ameren has agreed to place $100 million of the cash portion of the purchase price in a six-year escrow to secure payment of certain environmental indemnification obligations of Dynegy affiliates under the purchase agreement unless certain conditions are met prior to closing. Ameren's financing plan for this transaction includes the issuance of new Ameren common stock, which in total, is expected to equal at least 50% of the transaction value. We currently expect to issue common stock (including this offering of common stock) to finance the cash portion of the purchase price, to reduce Illinois Power debt assumed as part of this transaction and pay any related premiums and possibly to reduce present or future indebtedness and/or repurchase securities of Ameren or our subsidiaries.

        Upon completion of the acquisition, expected by the end of 2004, Illinois Power will become an Ameren subsidiary operating as AmerenIP. The transaction is subject to the approval of the Illinois Commerce Commission, or ICC, the Securities and Exchange Commission, or SEC, the Federal Energy Regulatory Commission, or FERC, the Federal Communications Commission, the expiration of the waiting period under the Hart-Scott-Rodino Act and other customary closing conditions.

        As noted, the transaction includes Dynegy's 20% ownership interest in EEI. Through our subsidiaries, we already have a 60% ownership interest in EEI. In addition, this transaction includes a firm capacity power supply contract for Illinois Power's annual purchase of 2,800 megawatts of electricity from a subsidiary of Dynegy. This contract will extend through 2006 and is expected to supply about 75% of Illinois Power's customer requirements.

        For the nine-month period ended September 30, 2003, Illinois Power had revenues of $1,190 million, operating income of $130 million, and net income applicable to common shareholder of $88 million, and at September 30, 2003 had total assets of $4.9 billion. For the year ended December 31, 2002, Illinois Power had revenues of $1,518 million, operating income of $164 million and net income applicable to common shareholder of $158 million and at December 31, 2002 had total assets of $4.9 billion. Illinois Power's total assets at both September 30, 2003 and December 31, 2002 included an intercompany note receivable from Illinova Corporation, its immediate parent company, of approximately $2.3 billion in principal amount that Illinois Power received in 1999 in connection with its transfer of its fossil-generating assets and other generation-related assets to Dynegy. Pursuant to the purchase agreement, this intercompany note will be eliminated prior to our acquisition of Illinois Power. We have obtained this information regarding Illinois Power's financial condition from its Quarterly Report on Form 10-Q for the quarter ended September 30, 2003 and filed with the SEC on November 14, 2003 and its Annual Report on Form 10-K for the year ended December 31, 2002 and filed with the SEC on April 15, 2003.

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The Offering

        All of the shares of our common stock offered hereby are being sold by us.

Common stock offered   19,063,181 shares

Common stock outstanding as of December 31, 2003
 
162,861,662 shares

Common stock outstanding as of December 31, 2003 adjusted for the offering
 
181,924,843 shares

New York Stock Exchange symbol
 
"AEE"

Common stock price range from January 1, 2002 to February 3, 2004
 
$34.72 to $48.34

Current indicated annual dividend rate
 
$2.54 per share

Use of proceeds
 
To provide funds required to pay the cash portion of the purchase price for our acquisition of Illinois Power and Dynegy's 20% interest in EEI and to reduce Illinois Power debt assumed as part of this transaction and pay any related premiums. Pending such use, and/or if the acquisition is not completed, we plan to use the net proceeds to reduce present or future indebtedness and/or repurchase securities of Ameren or our subsidiaries. A portion of the net proceeds may also be temporarily invested in short-term instruments.

Risk factors
 
You should carefully consider the information set forth under "Risk Factors" in this prospectus supplement before investing in our common stock.

        For a complete description of our common stock, please refer to the "Description of Common Stock" in the accompanying prospectus.

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Summary Financial Data

        The summary historical consolidated financial data of Ameren set forth below has been derived from (i) the annual consolidated financial statements of Ameren for the years 2000-2002, which have been audited by our independent accountants, and incorporated by reference in this prospectus supplement and the accompanying prospectus from Ameren's Annual Report on Form 10-K for the year ended December 31, 2002 (2002 Form 10-K), and (ii) the unaudited financial statements of Ameren for the nine months ended September 30, 2003 and 2002, incorporated by reference in this prospectus from Ameren's Quarterly Report on Form 10-Q for the quarter ended September 30, 2003, filed with the SEC on November 14, 2003 (September 2003 Form 10-Q). This information is qualified in its entirety by, and should be read in conjunction with, the consolidated financial statements, the notes thereto, and "Management's Discussion and Analysis of Financial Condition and Results of Operations" in the 2002 Form 10-K and the September 2003 Form 10-Q, incorporated by reference in this prospectus supplement and the accompanying prospectus. See "Where You Can Find More Information" in the accompanying prospectus.

 
   Nine Months Ended
September 30,
  
Year Ended December 31,
 
   2003(1)
   2002
   2002
   2001
   2000
 
   (unaudited)

    
    
    
 
   (in millions, except per share data)
Statement of Income Data:(2)                              
  Operating revenues   $ 3,546   $ 3,018   $ 3,841   $ 3,858   $ 3,856
  Operating expenses     2,595     2,151     2,968     2,893     2,915
  Operating income     951     867     873     965     941
  Income before cumulative effect of change in accounting principle     468     414     382     476     457
  Cumulative effect of change in accounting principle, net of income taxes     18     —     —     (7 )   —
  Net income     486     414     382     469     457
  Earnings per common share                              
    Basic     3.02     2.88     2.61     3.41     3.33
    Diluted     3.02     2.87     2.60     3.40     3.33

 
 
As of
September 30,
 
As of December 31,
 
   2003(1)
   2002
   2001
 
   (unaudited)

    
    
 
   (in millions)
Balance Sheet Data:                  
  Total assets   $ 13,537   $ 11,499   $ 10,401
  Long-term debt obligations(3)     4,046     3,433     2,835
  Preferred stock not subject to mandatory redemption     213     193     235
  Total stockholders' equity     4,334     3,842     3,349
(1)
Includes amounts for CILCORP and Medina Valley from the respective acquisition dates of January 31, 2003 and February 4, 2003.

(2)
Beginning with the September 2003 Form 10-Q, in order to be more consistent with industry reporting trends, our Statement of Income Data was reclassified to present all income taxes as one line item. Previously, we reported a portion of our income taxes in operating expenses and a portion in other income and deductions. This change results in our calculation of operating income now being on a pre-tax basis with no effect on net income. The information for the years ended 2000-2002 has been similarly reclassified in our Statement of Income Data in this prospectus supplement.

(3)
Excluding current maturities.

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RISK FACTORS

        In considering whether to purchase the common stock, you should carefully consider the following risk factors, which update and restate the risk factors contained in the accompanying prospectus, and therefore are intended to supersede those risk factors.

We may not be able to complete our acquisition of Illinois Power. If we do not complete the acquisition, dilution to our earnings per share will result unless we are able to otherwise use the proceeds from this offering so as to avoid or mitigate such dilution.

        On February 2, 2004, we entered into an agreement with Dynegy to purchase the stock of Illinois Power and Dynegy's 20% ownership interest in EEI. The total transaction value is approximately $2.3 billion, including the assumption of approximately $1.8 billion of Illinois Power debt and preferred stock. Ameren's financing plan for this transaction includes the issuance of new Ameren common stock, which in total, is expected to equal at least 50% of the transaction value. We currently expect to issue common stock (including this offering of common stock) to finance the cash portion of the purchase price, to reduce Illinois Power debt assumed as part of this transaction and pay any related premiums and possibly to reduce present or future indebtedness and/or repurchase securities of Ameren or our subsidiaries. The transaction is subject to various regulatory approvals, including the ICC, the SEC, the FERC, the Federal Communications Commission, the expiration of the waiting period under the Hart-Scott-Rodino Act and other customary closing conditions. Although we expect to complete the transaction by the end of 2004, we cannot be certain that all of the required approvals will be obtained, or the other closing conditions will be satisfied, within that time frame, if at all, or without terms and conditions that may have a material adverse effect on our operations. We are also relying on the ability of Dynegy to close the sale of Illinois Power when the required approvals are received. If we are unable to complete the acquisition, the issuance of the common stock offered by this prospectus supplement will result in dilution to our earnings per share unless we are able to otherwise use the proceeds from this offering in a manner that will avoid or mitigate such dilution.

If we are able to complete our acquisition of Illinois Power, we may not be able to successfully integrate it into our other businesses or achieve the benefits we anticipate.

        If we complete the acquisition of Illinois Power, we cannot assure you that we will be able to successfully integrate Illinois Power with our other businesses. The integration of Illinois Power with our other businesses will present us with significant challenges and, as a result, we may not be able to operate the combined company as effectively as we expect. We may also fail to achieve the anticipated benefits of the acquisition as quickly or as cost effectively as we anticipate or may not be able to achieve those benefits at all. While we expect that this acquisition will be accretive to earnings per share in the first full year of operation after the transaction is completed, this expectation is based on important assumptions, including assumptions related to interest rates and market prices for power, that may ultimately be incorrect. As a result, if we are unable to integrate our businesses effectively or achieve the benefits we anticipate, our financial position, results of operation and liquidity may be materially adversely affected.

The electric and gas rates that certain of our subsidiaries are allowed to charge in Missouri and Illinois are largely set through 2006. This "rate freeze", along with other actions of regulators, can significantly affect our earnings, liquidity and business activities and are largely outside our control.

        The rates that certain of our subsidiaries are allowed to charge for their services are the single most important item influencing our financial position, results of operations and liquidity. We are highly regulated and the regulation of the rates that our subsidiaries charge their customers is

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determined, in large part, outside of our control by governmental organizations, including the Missouri Public Service Commission, or Missouri PSC, the ICC and the FERC. We are also subject to regulation by the SEC under the Public Utility Holding Company Act of 1935. Decisions made by these regulators could have a material impact on our financial position, results of operations and liquidity.

        One of our primary operating subsidiaries is AmerenUE, which is the largest electric utility in Missouri. As a part of the settlement of AmerenUE's Missouri electric rate case in 2002, AmerenUE is subject to a rate moratorium providing for no changes in its electric rates in Missouri before June 30, 2006, subject to limited statutory and other exceptions. A rate reduction of $30 million will go into effect on April 1, 2004, which is the last portion of the $110 million rate reduction included in the stipulation entered into as part of the settlement of the Missouri electric rate case. In addition, as a provision of the Illinois legislation related to the restructuring of the Illinois electric industry, a rate freeze is in effect in Illinois through January 1, 2007. This Illinois legislation also contains a provision requiring that earnings from the Illinois jurisdiction in excess of certain levels be shared equally with AmerenUE's, AmerenCIPS' and AmerenCILCO's Illinois customers through 2006. This Illinois legislation is also applicable to Illinois Power. Furthermore, as part of the settlement of AmerenUE's Missouri gas rate case, which was approved by the Missouri PSC on January 13, 2004, AmerenUE agreed to a rate moratorium providing for no changes in its gas delivery rates prior to July 1, 2006, subject to certain exceptions (the increased rates approved as part of the settlement will become effective on February 15, 2004).

        As a part of the settlement of AmerenUE's Missouri electric rate case in 2002, AmerenUE also undertook to use commercially reasonable efforts to make critical energy infrastructure investments of $2.25 billion to $2.75 billion from January 1, 2002 through June 30, 2006, including, among other things, the addition of more than 700 megawatts of new generation capacity (240 megawatts of which was added in 2002) and the replacement of steam generators at AmerenUE's Callaway nuclear plant. The amount of energy infrastructure investment through June 2006 described in the settlement is consistent with AmerenUE's previously disclosed estimate of construction expenditures AmerenUE expects to make over the same time period. However, AmerenUE's agreement to a rate moratorium will result in these capital expenditures not becoming recoverable in rates, or earning a return, before June 30, 2006. Therefore, AmerenUE's undertakings with respect to making energy infrastructure investments and funding new programs, coupled with the rate reductions and rate moratorium described above, could result in increased financing requirements for AmerenUE and thus have a material impact on our liquidity.

        Our primary operating subsidiaries do not have the benefit of a fuel adjustment clause in either Missouri or Illinois for their electric operations that would allow them to recover increased fuel and power costs from customers. Therefore, to the extent that we have not hedged our fuel and power costs, we are exposed to changes in fuel and power prices to the extent fuel for our electric generating facilities and power must be purchased on the open market in order for us to serve our customers.

        Steps taken and being considered at the federal and state levels continue to change the structure of the electric industry and utility regulation. At the federal level, the FERC has been mandating changes in the regulatory framework in which transmission owning public utilities, such as AmerenUE, AmerenCIPS and AmerenCILCO operate. In Missouri, where a majority of our retail electric revenues are derived, restructuring bills have been introduced but no legislation has been passed. The Illinois Customer Choice Law provides for electric utility restructuring and retail direct access. Retail direct access, which allows customers to choose their electric generation supplier, was first offered to Illinois residential customers on May 1, 2002. Although retail direct access in Illinois has not had a negative effect on our revenues or liquidity, we expect competitive forces in the electric supply segment of our business to continue to increase.

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        The potential negative consequences associated with further electric industry restructuring in our service territories, if it occurs, could be significant and could include the impairment and writedown of certain assets, including generation related plant and net regulatory assets, lower revenues, reduced profit margins and increased costs of capital and operations expenses.

Increased federal and state environmental regulation could require us to incur large capital expenditures and increase our operating costs.

        Approximately 65% of our generating capacity is coal-fired. The balance is nuclear, gas-fired, hydro and oil-fired. The U.S. Environmental Protection Agency, or EPA, has recently issued proposed regulations with respect to sulfur dioxide (SO2), nitrogen oxide (NOx) and mercury emissions from coal-fired power plants. These new rules, if adopted, would require significant additional reductions in these emissions from our power plants in phases, beginning in 2010. The rules are currently under a public review and comment period, and may change before being issued as final late in 2004 or early 2005. Preliminary estimates of capital costs based on current technology on the Ameren systems to comply with the SO2 and NOx rules, as proposed, range from $400 million to $600 million by 2010, with an additional $500 million to $800 million by 2015. The proposed mercury regulations contain a number of options and the final control requirements are highly uncertain. Ameren anticipates additional capital costs to comply with the mercury rules could be up to $100 million by 2010. Depending upon the final mercury rules, similar additional costs could be incurred between 2010 and 2018.

        In addition, Illinois has developed a NOx control regulation for utility generating plant boilers consistent with an EPA program aimed at reducing ozone levels in the eastern United States. In February 2002, the EPA proposed similar rules for Missouri. We currently estimate that our remaining capital expenditures could range from $210 million to $250 million between 2004 and 2008 in order to comply with the final NOx regulations in Missouri and Illinois. This estimate includes the assumption that these rules will require the installation of selective catalytic reduction technology on some of our units, as well as additional controls. While costs incurred by AmerenUE would be eligible for recovery in rates, subject to Missouri PSC or ICC approval, as applicable, there is no similar mechanism for recovery of costs by any of our non rate-regulated generation businesses.

        We are unable to predict the ultimate effect of any new environmental regulations, guidelines, enforcement initiatives or legislation on our financial position, results of operations or liquidity. Any of these factors would add significant pollution control costs to our generating assets and therefore, could also increase financing requirements for certain of our operating subsidiaries.

Our required participation in a regional transmission organization could increase our costs, reduce our revenues and reduce our control over our transmission assets.

        In December 1999, the FERC issued Order 2000 requiring all utilities subject to FERC jurisdiction to state their intentions for joining a regional transmission organization, or RTO. Since April 2002, AmerenUE, AmerenCIPS and subsidiaries of FirstEnergy Corp. and NiSource Inc. (collectively, the GridAmerica Companies) have participated in a number of filings at the FERC in an effort to form GridAmerica LLC, or GridAmerica, as an independent transmission company. On December 19, 2002, the FERC issued an order conditionally approving the formation and operation of GridAmerica as an independent transmission company within the Midwest Independent System Operator, or MISO, subject to further compliance filings, which were made by the GridAmerica Companies in early 2003. AmerenCILCO is already a member of the MISO and has transferred functional control of its transmission system to the MISO. Transmission service on the AmerenCILCO transmission system is provided pursuant to the terms and conditions of the MISO Open Access Transmission Tariff, or OATT, on file with the FERC.

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        On April 30, 2003, the FERC issued an order authorizing the GridAmerica Companies' request to transfer functional control of their transmission assets to GridAmerica. The FERC also accepted the proposed rate amendments to the MISO OATT, filed in early 2003 by MISO and the GridAmerica Companies, effective upon the commencement of service over the GridAmerica transmission facilities under the MISO OATT, suspended the proposed rates for a nominal period, subject to refund, and established hearing and settlement judge procedures to determine the justness and reasonableness of the proposed rate amendments to the MISO OATT. In August 2003, the GridAmerica Companies filed acknowledgements with the FERC to permit GridAmerica to commence operations on October 1, 2003, on a phased basis, by assuming, with the MISO, functional control of the transmission systems of American Transmission Systems, Incorporated, a subsidiary of FirstEnergy Corp., and Northern Indiana Public Service Company, a subsidiary of NiSource Inc. Pursuant to this authorization, GridAmerica began operating on October 1, 2003.

        Also beginning on October 1, 2003, the proposed rates filed by MISO and the GridAmerica Companies became effective, subject to refund for FirstEnergy Corp. and NiSource Inc. Since AmerenUE and AmerenCIPS have not transferred functional control of their transmission assets to MISO, the proposed rates are not effective for AmerenUE or AmerenCIPS. Efforts to settle the disputed rate issues concerning rates for transmission service over the transmission assets of the GridAmerica Companies are continuing. AmerenUE's participation in GridAmerica is pending before the Missouri PSC for approval.

        Until the tariffs and other material terms of AmerenUE's and AmerenCIPS' participation in GridAmerica and GridAmerica's participation in the MISO are finalized and approved by the FERC and other regulatory authorities having jurisdiction, we are unable to predict the ultimate impact that ongoing RTO developments will have on our financial position, results of operations or liquidity. We could incur increased transmission related costs and reduced transmission service revenues, and we may be required to expand our transmission system according to decisions made by an RTO rather than our internal planning process once AmerenUE and AmerenCIPS begin participating in the MISO through GridAmerica.

The inability of AmerenUE and AmerenCIPS to recover "through and out" transmission revenues could result in a material net revenue reduction.

        On November 17, 2003, the FERC issued an order upholding an earlier order issued in July 2003 that will reduce AmerenUE's and AmerenCIPS', as well as other transmission-owning utilities' "through and out" transmission revenues effective April 1, 2004. The revenues subject to elimination by this order are those revenues from transmission reservations that travel through or out of AmerenUE's and AmerenCIPS' transmission system and are also used to provide electricity to load within the MISO or PJM Interconnection LLC systems. The magnitude of the potential net revenue reduction resulting from this order could be up to $20 million to $25 million annually if AmerenUE and AmerenCIPS are not in an RTO. While it is anticipated that AmerenUE's and AmerenCIPS' transmission revenues could be reduced by these orders, transmission expenses for Generating Company could be reduced. Moreover, the FERC's final order explicitly permits companies to collect the lost "through and out" revenues through other transitional rate mechanisms. Until it is determined when, or if, AmerenUE and AmerenCIPS will join an RTO, or the magnitude of lost "through and out" transmission revenue recovery AmerenUE and AmerenCIPS will receive through other rate mechanisms, AmerenUE and AmerenCIPS are unable to predict the ultimate impact of these orders.

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The substance and implementation of standard market design rules by the FERC is uncertain and may adversely affect the way in which we operate our transmission assets.

        On July 31, 2002, the FERC issued its standard market design notice of proposed rulemaking, or NOPR. The NOPR proposes a number of changes to the way the current wholesale transmission service and energy markets are operated. Specifically, the NOPR proposes that all jurisdictional transmission facilities be placed under the control of an independent transmission provider (similar to an RTO), proposes a new transmission service tariff that provides a single form of transmission service for all users of the transmission system including bundled retail load, and proposes a new energy market and congestion management system that uses locational marginal pricing as its basis. In our initial comments on the NOPR, which were filed at the FERC on November 15, 2002, we expressed our concern with the potential impact of the proposed rules in their current form on the cost and reliability of service to retail customers. We also proposed that certain modifications be made to the proposed rules in order to protect transmission owners from the possibility of trapped transmission costs that might not be recoverable from ratepayers as a result of inconsistent regulatory policies. We filed additional comments on the remaining sections of the NOPR during the first quarter of 2003.

        In April 2003, the FERC issued a "white paper" reflecting comments received in response to the NOPR. More specifically, the white paper indicated that the FERC will not assert jurisdiction over the transmission rate component of bundled retail service and will insure that existing bundled retail customers retain their existing transmission rights and retain rights for future load growth in its final rule. Moreover, the white paper acknowledged that the final rule will provide the states with input on resource adequacy requirements, allocation of firm transmission rights, and transmission planning. The FERC also requested input on the flexibility and timing of the final rule's implementation.

        Although issuance of the final rule is uncertain and its implementation schedule is still unknown, the MISO is already in the process of implementing a separate market design similar to the proposed market design in the NOPR. In July 2003, the MISO filed with the FERC a revised OATT codifying the terms and conditions under which it will implement the new market design. Thereafter, on October 17, 2003, the MISO filed a motion to withdraw its revised OATT. On October 29, 2003, the FERC issued a series of orders granting the motion for withdrawal of the revised OATT and providing guidance to be followed by the MISO in developing a new energy market design in the future. Until the FERC issues a final rule and the MISO finalizes its new market design, we are unable to predict the ultimate impact of the NOPR or the MISO new market design on our future financial position, results of operations or liquidity.

Increasing costs associated with our defined benefit retirement plans, healthcare plans and other employee related benefits may adversely affect our results of operations, liquidity and financial position.

        We made cash contributions totaling $25 million and $31 million to our defined benefit retirement plans during 2003 and 2002, respectively. In addition, at December 31, 2002, we recorded a minimum pension liability of $102 million after taxes which resulted in a charge to other comprehensive income and a reduction in stockholders' equity. We expect to be required under the Employee Retirement Income Security Act of 1974 to fund an average of approximately $115 million annually from 2005 through 2008, in order to maintain minimum funding levels for our pension plans, assuming the passage of a law which would be retroactive to January 1, 2004 to extend the temporary interest rate relief used to calculate pension liabilities in 2002 and 2003, that expired on December 31, 2003. These amounts are estimates and may change based on actual stock market performance, changes in interest rates, and any pertinent changes in government regulations, each of which could also result in a requirement to record an additional minimum pension liability.

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Furthermore, if we complete our acquisition of Illinois Power, we could incur material funding requirements with respect to Illinois Power's existing defined benefit retirement plans.

        In addition to the costs of our retirement plans, the costs to us of providing health-care benefits to our employees and retirees have increased substantially in recent years. In 2003 and 2002 our costs of providing health-care benefits to our employees and retirees increased approximately 14% and 15%, respectively, in each case as compared with the preceding year.

        We believe that our employee benefit costs, including costs related to healthcare plans for our employees and former employees, will continue to rise. The increasing costs and funding requirements associated with our defined benefit retirement plans, healthcare plans and other employee benefits may adversely affect our results of operations, liquidity or financial position.

Our electric generating facilities are subject to operational risks that could result in unscheduled plant outages, unanticipated operation and maintenance expenses and increased power purchase costs.

        Our subsidiaries, AmerenUE, AmerenCILCO, Generating Company, AERG, Medina Valley, and EEI own and operate coal, nuclear, gas-fired, hydro and oil-fired generating facilities constituting approximately 14,500 megawatts (net) of installed capability. Operation of electric generating facilities involves certain risks which can adversely affect energy output and efficiency levels. Included among these risks are:

A substantial portion of our non rate-regulated generating capacity is committed under affiliate contracts which expire over the next several years.

        We have several electric power supply agreements under which subsidiaries of ours that own generating facilities (or market their output) directly or indirectly supply the full requirements of our rate-regulated utility subsidiaries, including the following:

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        We currently plan to pursue renewals or extensions of these full requirements agreements as they expire. Such renewals or extensions will depend on compliance with federal and state regulatory requirements in effect at the time. Extensions through December 31, 2006 of the agreements to which AmerenCIPS and AmerenCILCO are a party have been required by the ICC in its order approving our acquisition of CILCORP and AmerenCILCO.

        Midwest power markets have experienced high levels of new capacity development over the last several years, which, in part, has contributed to soft long-term power prices in this region. Owners of generating capacity in the Midwest are actively seeking markets for their energy and capacity and have asked our regulators to closely scrutinize power supply arrangements among our subsidiaries when we have sought approval to enter into them. Even though the ICC has required those extensions, we cannot predict whether we will be successful in obtaining extensions of the agreements described above when they expire. To the extent we cannot secure extensions or other long-term replacement power sale contracts for the energy and capacity currently committed under these agreements, our generating subsidiaries and Marketing Company will face competition from other power suppliers in the Midwest and will be exposed to price risk.

        Generating Company participates with AmerenUE in an agreement to jointly dispatch its generating facilities with those of AmerenUE, which thereby produces benefits and efficiencies for both generating parties. Pending or future federal and state regulatory proceedings and policies may evolve in ways that could impact Generating Company's ability to continue to participate in these affiliate transactions on current terms.

Our non rate-regulated electric generating facilities must compete for the sale of energy and capacity, which exposes us to price risk.

        As owners of non rate-regulated electric generating facilities, Generating Company (4,800 megawatts) and AERG (1,100 megawatts) will not have any recovery of their costs or any specified rate of return set by a regulatory body. Of these non rate-regulated electric generating facilities, approximately 3,500 megawatts are currently under full requirements contracts with our affiliates, including the contracts referred to in the immediately preceding risk factor. The remainder of the generating capacity must compete for the sale of energy and capacity. AmerenUE is currently seeking regulatory approval of the transfer by Generating Company to it of approximately 550 megawatts of combustion turbine generating units at Pinckneyville and Kinmundy, Illinois, which transfer we expect to occur in 2004, with the result that those generating units will no longer be non rate-regulated.

        To the extent electric capacity generated by these facilities is not under contract to be sold, either now or in the future, the revenues and results of operations of these non rate-regulated subsidiaries will generally depend on the prices that they can obtain for energy and capacity in Illinois and adjacent markets. Among the factors that could influence such prices (all of which are beyond our control to a significant degree) are:

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Our ownership and operation of a nuclear generating facility creates business, financial and waste disposal risks.

        Through AmerenUE, we own the Callaway nuclear plant, which represents approximately 14% of AmerenUE's generation capability. We are, therefore, subject to the risks of nuclear generation, which include the following:


        The Nuclear Regulatory Commission, or NRC, has broad authority under federal law to impose licensing and safety related requirements for the operation of nuclear generation facilities. In the event of non-compliance, the NRC has the authority to impose fines or shut down a unit, or both, depending upon its assessment of the severity of the situation, until compliance is achieved. Revised safety requirements promulgated by the NRC could necessitate substantial capital expenditures at nuclear plants such as ours. In addition, although we have no reason to anticipate a serious nuclear incident at our plant, if an incident did occur, it could harm our results of operations or financial position. A major incident at a nuclear facility anywhere in the world could cause the NRC to limit or prohibit the operation or licensing of any domestic nuclear unit.

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Our energy risk management strategies may not be effective in managing fuel and electricity pricing risks, which could result in unanticipated liabilities to us or increased volatility of our earnings.

        We are exposed to changes in market prices for natural gas, fuel, electricity and emission credits. Prices for natural gas, fuel, electricity and emission credits may fluctuate substantially over relatively short periods of time and expose us to commodity price risk. We utilize derivatives such as forward contracts, futures contracts, options and swaps to manage these risks. We attempt to manage our exposure from these activities through enforcement of established risk limits and risk management procedures. We cannot assure you that these strategies will be successful in managing our pricing risk, or that they will not result in net liabilities to us as a result of future volatility in these markets.

        In addition, although we routinely enter into contracts to offset our positions (i.e., to hedge our exposure to the risks of demand, market effects of weather and changes in commodity prices), we do not always hedge the entire exposure of our operations from commodity price volatility. Furthermore, our ability to hedge our exposure to commodity price volatility depends on liquid commodity markets. As a result, to the extent the commodity markets are illiquid, we may not be able to execute our risk management strategies, which could result in greater open positions than we would prefer at a given time. To the extent that open positions exist, fluctuating commodity prices can improve or diminish our financial results and financial position.

Our businesses are dependent on our ability to successfully access the capital markets. We may not have access to sufficient capital in the amounts and at the times needed.

        We rely on access to short-term and long-term capital markets as a significant source of liquidity and funding for capital requirements not satisfied by our operating cash flows. The inability to raise capital on favorable terms, particularly during times of uncertainty in the capital markets, could negatively impact our ability to maintain and grow our businesses. Based on our current credit ratings, we believe that we will continue to have access to the capital markets. However, events beyond our control may create uncertainty in the capital markets such that our cost of capital would increase or our ability to access the capital markets would be adversely affected.


USE OF PROCEEDS

        The net proceeds to be received by us from this offering, after deducting the underwriter's discount and estimated expenses, are estimated to be approximately $852.6 million. We anticipate using the net proceeds from this offering of our common stock to provide funds required to pay the cash portion of the purchase price for our acquisition of Illinois Power and Dynegy's 20% interest in EEI and to reduce Illinois Power debt assumed as part of this transaction and pay related premiums. Pending such use, and/or if the acquisition is not completed, we plan to use the net proceeds to reduce present or future indebtedness and/or repurchase securities of Ameren or our subsidiaries. A portion of the net proceeds may also be temporarily invested in short-term instruments.

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CAPITALIZATION

        The following table sets forth our actual capitalization and short-term debt as of September 30, 2003 and as adjusted (1) to give effect to certain financing activity during the 4th quarter of 2003 including (a) the issuance by AmerenUE of $200 million principal amount of 4.65% Senior Secured Notes due 2013 in October 2003, (b) the maturity of $150 million of our Floating Rate Notes due 2003 in December 2003, (c) the redemption by AmerenCIPS of $30 million of its auction preferred stock in December 2003 and (d) an amortization payment of $14 million in December 2003, with respect to EEI's medium-term notes, and (2) to give effect to this offering of 19,063,181 shares of our common stock but without giving effect to the pending acquisition of Illinois Power or use of proceeds in connection therewith.

        From time to time, we may issue additional debt or equity securities. The following information is qualified in its entirety by and should be read in conjunction with our consolidated financial statements, including the notes thereto, which are incorporated by reference in this prospectus supplement and the accompanying prospectus. See "Where You Can Find More Information" in the accompanying prospectus.

 
   As of September 30, 2003
  
 
   Actual
   As Adjusted
To Reflect
4th Quarter
Financing
Activity
   As Adjusted
To Reflect
4th Quarter
Financing
Activity and
This Offering of
Common Stock
   Percentage
As Adjusted
  
 
   (in millions of dollars)

    
  
Short-term debt, including current maturities of long-term debt   $ 502   $ 338   $ 338   3.4 %
Long-term debt (net), less current maturities     4,046     4,246     4,246   42.6  
Preferred stock subject to mandatory redemption     21     21     21   0.2  
Preferred stock not subject to mandatory redemption     213     183     183   1.8  
Total stockholders' equity     4,334     4,334     5,187   52.0  
   
 
 
 
 
  Total capitalization   $ 9,116   $ 9,122   $ 9,975   100.0 %
   
 
 
 
 

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PRICE RANGE OF COMMON STOCK AND DIVIDENDS

        Our common stock is listed and traded on the New York Stock Exchange under the symbol "AEE." The following table provides, for the calendar quarters indicated, the composite high and low prices per share for the periods shown below as reported and the amount of per-share dividends paid in the periods indicated.

Period
   High
   Low
   Dividends
Paid
2002:                  
First Quarter   $ 43.85   $ 39.50   $ 0.635
Second Quarter     45.20     40.20     0.635
Third Quarter     45.14     34.72     0.635
Fourth Quarter     42.69     38.75     0.635

2003:
 
 
 
 
 
 
 
 
 
First Quarter   $ 44.73   $ 37.43   $ 0.635
Second Quarter     46.50     38.89     0.635
Third Quarter     44.80     40.74     0.635
Fourth Quarter     46.17     42.55     0.635

2004:
 
 
 
 
 
 
 
 
 
First Quarter (through February 3, 2004)   $ 48.34   $ 44.91     —

        On February 3, 2004, the last reported sale price of our common stock on the New York Stock Exchange was $47.57. As of December 31, 2003, there were approximately 90,284 holders of record of our common stock. See "Description of Common Stock — Dividend Rights and Limitations" in the accompanying prospectus.

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UNDERWRITING

        Ameren and Goldman, Sachs & Co., as underwriter, have entered into an underwriting agreement with respect to the shares being offered. Subject to certain conditions, Goldman, Sachs & Co. has agreed to purchase all of the 19,063,181 shares offered hereby.

        Shares sold by Goldman, Sachs & Co. to the public will initially be offered at the initial public offering price set forth on the cover of this prospectus supplement. In addition, Goldman, Sachs & Co. may receive from purchasers of the shares normal brokerage commissions in amounts agreed upon with such purchasers. If all the shares are not sold at the initial price to public, Goldman, Sachs & Co. may change the offering price and the other selling terms.

        Ameren has agreed for a period of 90 days, subject to certain exceptions, and its senior executive officers have agreed for a period of 90 days (except for an aggregate of up to 12,790 shares of common stock), not to offer, sell, contract to sell or otherwise dispose of, directly or indirectly, or file with the SEC a registration statement under the Securities Act of 1933 relating to, shares of its common stock, securities convertible into or exchangeable or exercisable for any shares of its common stock, enter into a transaction that would have the same effect, or enter into any swap, hedge or other arrangement that transfers, in whole or in part, any of the economic consequences of ownership of its common stock, without the prior written consent of Goldman, Sachs & Co.

        In connection with the offering, Goldman Sachs & Co. may purchase and sell shares of common stock in the open market. These transactions may include short sales, stabilizing transactions and purchases to cover positions created by short sales. Short sales involve the sale by Goldman, Sachs & Co. of a greater number of shares than it is required to purchase in the offering. Goldman, Sachs & Co. will need to close out any short sale by purchasing shares in the open market. Goldman, Sachs & Co. is likely to create a short position if it is concerned that there may be downward pressure on the price of the common stock in the open market after pricing that could adversely affect investors who purchase in the offering. Stabilizing transactions consist of various bids for or purchases of common stock made by Goldman Sachs & Co. in the open market prior to the completion of the offering.

        Purchases to cover a short position and stabilizing transactions may have the effect of preventing or retarding a decline in the market price of the common stock, and may stabilize, maintain or otherwise affect the market price of the common stock. As a result, the price of the common stock may be higher than the price that otherwise might exist in the open market. If these activities are commenced, they may be discontinued at any time. These transactions may be effected on the New York Stock Exchange, in the over-the-counter market or otherwise.

        Ameren estimates that its share of the total expenses of this offering, excluding underwriting discounts will be approximately $400,000.

        Ameren has agreed to indemnify Goldman, Sachs & Co. against certain liabilities, including liabilities under the Securities Act of 1933.

        From time to time, Goldman, Sachs & Co. and certain of its affiliates have engaged, and may in the future engage, in transactions (including investment banking and commercial banking transactions) with, and perform services for, Ameren and its affiliates in the ordinary course of business. Goldman, Sachs & Co. is acting as financial advisor to Ameren in connection with the proposed acquisition of Illinois Power, for which it will receive customary compensation. An affiliate of Goldman, Sachs & Co. is a lender under certain credit facilities for Ameren.

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LEGAL MATTERS

        The validity of the shares will be passed upon for Ameren by Steven R. Sullivan, Esq., our Senior Vice President Governmental/Regulatory Policy, General Counsel and Secretary, and by Thelen Reid & Priest LLP, New York, New York. Certain legal matters relating to the shares will be passed upon for the underwriter by Pillsbury Winthrop LLP, New York, New York. Pillsbury Winthrop LLP represents Ameren and its subsidiaries from time to time in connection with various matters.


EXPERTS

        The consolidated financial statements incorporated in this prospectus supplement and the accompanying prospectus by reference to our Annual Report on Form 10-K, as amended by Form 10-K/A, for the year ended December 31, 2002 have been so incorporated in reliance upon the report of PricewaterhouseCoopers LLP, independent accountants, given on the authority of said firm as experts in auditing and accounting.

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PROSPECTUS

LOGO

$1,472,875,000

AMEREN CORPORATION

Senior Debt Securities
Subordinated Debt Securities
Common Stock
Stock Purchase Contracts
Stock Purchase Units

AMEREN CAPITAL TRUST I
AMEREN CAPITAL TRUST II
Trust Preferred Securities
Guaranteed as set forth herein by Ameren Corporation

        Ameren Corporation, Ameren Capital Trust I and Ameren Capital Trust II intend to offer these securities from time to time in one or more series with an aggregate offering price not to exceed $1,472,875,000. This prospectus provides you with a general description of these securities. We will provide specific information about the offering and the terms of these securities in supplements to this prospectus. The supplements may also add, update or change information contained in this prospectus. You should read this prospectus and the supplements carefully before investing. This prospectus may not be used to sell any of these securities unless accompanied by a prospectus supplement.

        The common stock of Ameren Corporation is listed on the New York Stock Exchange under the symbol "AEE."

        See "Risk Factors" beginning on page 3 to read about certain factors you should consider.

        Our principal executive offices are located at 1901 Chouteau Avenue, St. Louis, Missouri 63103 and our telephone number is (314) 621-3222.

        Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

        We and the trusts may offer these securities directly or through underwriters, agents or dealers. Each prospectus supplement will provide the terms of the plan of distribution relating to each series of securities. See "Plan of Distribution."

The date of this prospectus is August 14, 2002.


TABLE OF CONTENTS

 
   Page
Risk Factors   3

Ameren Corporation
 
9

Ameren Capital Trust I and Ameren Capital Trust II
 
10

Where You Can Find More Information
 
11

Ratios of Earnings to Fixed Charges
 
12

Use of Proceeds
 
12

Forward-Looking Statements
 
13

About This Prospectus
 
15

Description of Debt Securities
 
16

Description of Trust Preferred Securities
 
26

Description of Guarantees
 
41

Relationship Among Trust Preferred Securities, Debt Securities and Guarantees
 
44

Description of Common Stock
 
46

Description of Stock Purchase Contracts and Stock Purchase Units
 
48

Book-Entry System
 
49

Plan of Distribution
 
52

Legal Matters
 
53

Experts
 
53


RISK FACTORS

        In considering whether to purchase any of the securities being offered, you should carefully consider all the information we have included or incorporated by reference in this prospectus. In particular, you should carefully consider the risk factors described below, as well as the factors listed in "Forward-Looking Statements."

The terms of AmerenUE's stipulation to settle its Missouri excess earnings complaint case will result in a charge to earnings, rate reductions and potentially increased financing requirements.

        One of our primary operating subsidiaries is Union Electric Company, or AmerenUE, which is the largest electric utility in Missouri. After AmerenUE's experimental alternative regulation plan for its Missouri retail electric customers expired on June 30, 2001, the Missouri PSC Staff filed an excess earnings complaint against AmerenUE with the Missouri PSC in July 2001. On March 1, 2002, the Missouri PSC Staff filed a recommendation that AmerenUE reduce its annual Missouri electric revenues by $246 million to $285 million. The Staff's recommendation was based on a return to traditional cost of service ratemaking, a lowered return on equity, a reduction in AmerenUE's depreciation rates and other cost of service adjustments. Hearings on the Staff's complaint began on July 11, 2002.

        On July 16, 2002, AmerenUE, the Missouri PSC Staff and all of the other parties to the proceeding submitted to the Missouri PSC a stipulation and agreement resolving this case. The Missouri PSC approved the stipulation and agreement on July 25, 2002. The stipulation and agreement includes an alternative rate regulation plan whose principal features are:

        As a result of the stipulation and agreement, we expect our earnings to be reduced by approximately $0.22 per share in 2002 as a result of the program funding obligations AmerenUE has committed to make over the next four years and the annualized impact of the 2002 electric rate reduction at AmerenUE, partially offset by the decrease in AmerenUE's annual depreciation expense. The amount of energy infrastructure investment through June 2006 described in the stipulation and agreement is consistent with our previously-disclosed estimate of construction expenditures AmerenUE expects to make over the same time period. However, AmerenUE's agreement to a rate moratorium would result in these capital expenditures not becoming recoverable in rates, or earning a return, before June 30, 2006. Therefore, AmerenUE's undertakings with respect to making energy

3


infrastructure investments and funding new programs, coupled with the rate reductions and rate moratorium described above, could result in increased financing requirements for AmerenUE. Since the stipulation and agreement permits AmerenUE to purchase a portion of its new regulated generating capacity from other Ameren companies at net book value, we expect a portion of the net proceeds of any such increased financings to be retained within the Ameren system and applied to our overall capital requirements.

        One of the rating agencies who assign ratings to our outstanding securities and those of our publicly-rated subsidiaries has not yet determined whether the stipulation and agreement would necessitate any changes to its outstanding ratings of these securities. Any adverse change in our securities ratings resulting from the stipulation and agreement could increase our cost of funds in our capital raising efforts.

Increased federal and state environmental regulation could require us to incur large capital expenditures and increase our operating costs.

        Approximately 67% of our generating capacity is coal-fired. The balance is nuclear, gas-fired, hydro and oil-fired. The U.S. Environmental Protection Agency is currently working on new ambient standards with respect to sulphur dioxide (SO2) and nitrogen oxide (NOx) emissions as well as on regulations and guidelines to regulate mercury emissions and air pollution from coal-fired power plants. In addition, the U.S. Congress has been working on legislation to consolidate the numerous air pollution regulations facing the utility industry. There is significant uncertainty with respect to the content of these regulations, guidelines and legislation.

        The State of Illinois has developed a NOx control regulation for utility generating plant boilers consistent with an EPA program aimed at reducing ozone levels in the eastern United States. In February 2002, the EPA proposed similar rules for Missouri. We currently estimate that our capital expenditures to comply with the final NOx regulations in Missouri and Illinois could range from $300 million to $350 million.

        We cannot predict the ultimate effect of any new environmental regulations, guidelines or legislation on our financial condition, results of operations and liquidity. Any of these regulations, guidelines or legislation could add significant pollution control costs to our generating assets, although perhaps not until later in the decade.

We may not be able to complete our acquisition of CILCORP or to successfully integrate it into our businesses.

        On April 28, 2002, we entered into an agreement with The AES Corporation to purchase all of the outstanding stock of CILCORP Inc., the parent company of Peoria-based Central Illinois Light Company, which operates as CILCO. We also agreed to acquire AES Medina Valley (No. 4), L.L.C. which indirectly owns a 40 megawatt, gas-fired electric generation plant. The total purchase price is approximately $1.4 billion, subject to adjustment for changes in CILCORP's working capital, and includes the assumption of CILCORP and AES Medina Valley debt at closing, estimated at approximately $900 million, with the balance of the purchase price in cash. We currently expect to finance a significant portion of the cash component of the purchase price through the issuance of common equity. The transaction is subject to various regulatory approvals, including the approval of the Illinois Commerce Commission, the SEC, the Federal Energy Regulatory Commission, the expiration of the waiting period under the Hart-Scott-Rodino Act and other closing conditions. Although we expect to complete the transaction by March 2003, we cannot be certain that all of the required approvals will be obtained, or the other closing conditions will be satisfied, within that time frame, if at all, or without terms and conditions that may have a material adverse effect on our operations. We are also relying on

4


the continued financial stability of The AES Corporation, and on its ability to close the sale of CILCORP when the required approvals are received. In addition, if we complete the acquisition, we cannot assure you that we will be able to successfully integrate CILCO with our other businesses. If we are unable to complete the acquisition or to integrate our businesses effectively, our financial position, results of operation and liquidity may be materially adversely affected.

Our required participation in a regional transmission organization could increase our costs and reduce our control over our transmission assets.

        In December 1999, the Federal Energy Regulatory Commission issued Order 2000 requiring all utilities subject to FERC jurisdiction, to state their intentions for joining a regional transmission organization or RTO. Since January 2001, we, along with several other utilities, have been seeking approval from the FERC to participate in the Alliance RTO. In late 2001, the FERC issued an order that rejected the formation of the Alliance RTO. In that same order, the FERC granted RTO status to the Midwest Independent System Operator or MISO, of which we had previously been a member, and ordered the Alliance RTO companies and the MISO to discuss how the Alliance RTO model could be accommodated within the MISO. The Alliance RTO and MISO were unable to reach such an agreement.

        In April 2002, the FERC ordered the Alliance companies to select the RTO in which they will participate within 30 days of the order. On May 28, 2002, AmerenUE and Central Illinois Public Service Company, also a subsidiary of Ameren (referred to collectively as the Ameren companies), made a filing with the FERC indicating that we would return to the MISO, although the exact nature of our participation was subject to additional negotiations. On June 20, 2002, the Ameren companies joined with subsidiaries of FirstEnergy Corp. and NiSource Inc. in filing with the FERC a letter of intent to form a new independent transmission company, GridAmerica LLC, that would join the MISO. Pending FERC approval of the definitive agreements establishing GridAmerica, National Grid will serve as the managing member of GridAmerica and will manage the transmission assets of the three companies and participate in the MISO on behalf of GridAmerica. Other Alliance RTO companies announced their intentions to join the Pennsylvania-Jersey-Maryland (PJM) RTO. On July 25, 2002, the Ameren companies filed a motion with the FERC requesting that it condition its approval of the choices of other Illinois utilities to join the PJM RTO on MISO and PJM entering into an agreement addressing important reliability and rate barrier issues. On July 31, 2002, the FERC issued an order accepting the formation of GridAmerica as an independent transmission company under the MISO subject to further compliance filings ordered by the FERC. The FERC also issued an order accepting the elections made by the other Illinois utilities to join the PJM RTO on the condition PJM and MISO immediately begin a process to address the reliability and rate-barrier issues raised by us and other market participants in previous filings.

        Until the "seams" issues are resolved as ordered by the FERC, and the tariffs and other material terms of our participation in GridAmerica and GridAmerica's participation in the MISO are finalized and approved by the FERC, we cannot predict whether we will in fact become a member of GridAmerica or MISO, or the impact that ongoing RTO developments will have on our financial condition, results of operation or liquidity. Our revenues from customers to whom we currently provide transmission services could decrease, we will incur fees and increased costs to participate in an RTO and we may be required to expand our transmission system according to decisions made by an RTO rather than our internal planning process.

5


Our electric generating facilities are subject to operational risks that could result in unscheduled plant outages, unanticipated operation and maintenance expenses and increased power purchase costs.

        AmerenUE owns and operates coal, nuclear, gas-fired, hydro and oil-fired generating facilities constituting approximately 8.3 million of net kilowatt installed capability. In addition, we own and operate AmerenEnergy Generating Company, a non-regulated wholesale electricity supplier. AmerenEnergy Generating owns and operates coal, gas-fired and oil-fired generating facilities constituting approximately 4.3 million of net kilowatt installed capability. Operation of electric generating facilities involves certain risks which can adversely affect energy output and efficiency levels. Included among these risks are:

        In addition, as a non-regulated owner of electric generating facilities, AmerenEnergy Generating will not have any recovery of its costs or any specified rate of return set by a regulatory body. Therefore, its revenues and results of operations will generally depend on the prices that AmerenEnergy Generating can obtain for energy and capacity in Illinois and adjacent markets. Among the factors that could influence such prices (all of which are beyond our control to a significant degree) are:

6

Our ownership and operation of a nuclear generating facility creates business, financial and waste disposal risks.

        Through AmerenUE, we own the Callaway Nuclear Plant, which represents approximately 14% of AmerenUE's generation capability. We are, therefore, subject to the risks of nuclear generation, which include the following:

        The Nuclear Regulatory Commission has broad authority under federal law to impose licensing and safety-related requirements for the operation of nuclear generation facilities. In the event of non-compliance, the NRC has the authority to impose fines or shut down a unit, or both, depending upon its assessment of the severity of the situation, until compliance is achieved. Revised safety requirements promulgated by the NRC could necessitate substantial capital expenditures at nuclear plants such as ours. In addition, although we have no reason to anticipate a serious nuclear incident at our plant, if an incident did occur, it could harm our results of operations or financial condition. A major incident at a nuclear facility anywhere in the world could cause the NRC to limit or prohibit the operation or licensing of any domestic nuclear unit.

Our energy trading and risk management strategies may not be effective in managing fuel and electricity pricing risks, which could result in unanticipated liabilities to us or increased volatility of our earnings.

        We are exposed to changes in market prices for natural gas, fuel, electricity and emission credits. Prices for natural gas, fuel, electricity and emission credits may fluctuate substantially over relatively short periods of time and expose us to commodity price risk. We utilize derivatives such as forward contracts, futures contracts, options and swaps to manage these risks. We attempt to manage our exposure from these activities through enforcement of established risk limits and risk management procedures. We cannot assure you that these strategies will be successful in managing our pricing risk, or that they will not result in net liabilities to us as a result of future volatility in these markets.

        In addition, although we routinely enter into contracts to offset our positions (i.e., to hedge our exposure to the risks of demand, market effects of weather and changes in commodity prices), we do not always hedge the entire exposure of our operations from commodity price volatility. To the extent that open positions exist, fluctuating commodity prices can improve or diminish our financial results and financial position.

The actions of regulators can significantly affect our earnings, liquidity and business activities and are largely outside our control.

        The rates that certain of our subsidiaries are allowed to charge for their services are the single most important item influencing our financial position, results of operations and liquidity. We are highly regulated and the regulation of the rates that our subsidiaries charge their customers is determined, in large part, outside of our control by governmental organizations, including the Missouri

7


PSC, the Illinois Commerce Commission and the FERC. We are also subject to regulation by the SEC under the Public Utility Holding Company Act of 1935. Decisions made by these regulators could have a material impact on our financial position, results of operations and liquidity.

        Steps taken and being considered at the federal and state levels continue to change the structure of the electric industry and utility regulation. At the federal level, the FERC has been mandating changes in the regulatory framework in which transmission-owning public utilities, such as AmerenUE and AmerenCIPS, operate. In Missouri, where approximately 70% of our retail electric revenues are derived, restructuring bills have been introduced but no legislation has been passed. Furthermore, no restructuring legislation was passed by the Missouri state legislature in 2002. In Illinois, the Electric Service Customer Choice and Rate Relief Law of 1997 provides for electric utility restructuring and the phasing-in through 2002 of retail direct access. Retail direct access, which allows customers to choose their electric generation supplier, was first offered to Illinois residential customers on May 1, 2002. Although retail direct access in Illinois has not had a negative effect on our revenues or liquidity and has not yet been introduced in Missouri, we expect competitive forces in the electric supply segment of our business to continue to increase.

        The potential negative consequences associated with further electric industry restructuring in our service territories, if it occurs, could be significant and could include the impairment and writedown of certain assets, including generation-related plant and net regulatory assets, lower revenues, reduced profit margins and increased costs of capital and operations expenses.

8


AMEREN CORPORATION

        Ameren is a public utility holding company registered under the Public Utility Holding Company Act of 1935 and does not own or operate any significant assets other than the stock of its subsidiaries. We were incorporated in Missouri on August 7, 1995. On December 31, 1997, following the receipt of all required approvals, CIPSCO Incorporated and AmerenUE combined with the result that the common shareholders of CIPSCO and AmerenUE became the common shareholders of Ameren, and Ameren became the owner of 100% of the common stock of AmerenUE and CIPSCO's subsidiaries, Central Illinois Public Service Company, or AmerenCIPS, and CIPSCO Investment Company.

        Our primary operating subsidiaries are AmerenUE, AmerenCIPS and AmerenEnergy Generating Company, which collectively are engaged principally in the generation, transmission, distribution and sale of electric energy and the purchase, distribution, transportation and sale of natural gas. AmerenUE, a first tier subsidiary, was incorporated in Missouri in 1922 and is successor to a number of companies, the oldest of which was organized in 1881. It is the largest electric utility in the State of Missouri and supplies electric and gas service in territories in Missouri and Illinois having an estimated population of 2.6 million within an area of approximately 24,500 square miles, including the greater St. Louis area. AmerenUE supplies electric service to about 1.2 million customers and natural gas service to about 130,000 customers. AmerenCIPS, also a first tier subsidiary, is an Illinois corporation organized in 1902. It supplies electric and gas utility service to territories in central and southern Illinois having an estimated population of 820,000 within an area of approximately 20,000 square miles. AmerenCIPS supplies electric service to about 325,000 customers and natural gas service to about 170,000 customers. AmerenEnergy Generating Company, an indirect wholly owned nonregulated electric generating subsidiary of AmerenEnergy Resources Company, was incorporated in Illinois in March 2000 in conjunction with the Illinois Electric Service Customer Choice and Rate Relief Law of 1997. This law provides for electric utility restructuring and introduces competition into the supply of electric energy at retail in Illinois. AmerenEnergy Generating Company, a wholesale electricity supplier, commenced operations on May 1, 2000 when AmerenCIPS transferred all of its net electric generating assets to it at net book value, consisting of certain generating facilities, all related fuel, supply, transportation, maintenance and labor agreements, approximately 45% of AmerenCIPS' employees, and some other related rights, assets and liabilities.

        We, directly or indirectly, also own all of the common stock of the following principal subsidiary companies:

        In addition, we indirectly own 60% of the common stock of Electric Energy, Inc., which owns and/or operates electric generation and transmission facilities in Illinois that supply electric power primarily to a uranium enrichment plant located in Paducah, Kentucky.

9


        On April 28, 2002, we entered into an agreement with The AES Corporation to purchase all of the outstanding stock of CILCORP Inc. CILCORP is the parent company of Peoria-based Central Illinois Light Company, which operates as CILCO. We also agreed to acquire AES Medina Valley (No. 4), L.L.C. which indirectly owns a 40 megawatt, gas-fired electric generation plant. The total purchase price is approximately $1.4 billion, subject to adjustment for changes in CILCORP's working capital, and includes the assumption of CILCORP and AES Medina Valley debt at closing, estimated at approximately $900 million, with the balance of the purchase price in cash. We currently expect to finance a significant portion of the cash component of the purchase price through the issuance of new common equity. The purchase will include CILCORP's regulated natural gas and electric businesses in Illinois serving approximately 200,000 and 205,000 customers, respectively, of which approximately 150,000 are combination electric and gas customers. In addition, the purchase includes approximately 1,200 megawatts of largely coal-fired generating capacity most of which is expected to be nonregulated by closing. Upon completion of the acquisition, expected by March 2003, CILCO will become an Ameren subsidiary, but will remain a separate utility company, operating as AmerenCILCO. The transaction is subject to the approval of the Illinois Commerce Commission, the SEC, the FERC, the expiration of the waiting period under the Hart-Scott-Rodino Act and other customary closing conditions.

        In this prospectus, "Ameren," "we," "us" and "our" refer to Ameren Corporation and, unless the context otherwise indicates, do not include our subsidiaries.


AMEREN CAPITAL TRUST I AND AMEREN CAPITAL TRUST II

        Ameren Capital Trust I and Ameren Capital Trust II are identical Delaware business trusts formed to permit us to raise capital by issuing trust preferred securities under this prospectus and a prospectus supplement, and investing the proceeds in debt securities issued by us.

        We have designated The Bank of New York (Delaware), as trustee for each of the trusts, in the State of Delaware for the purpose of complying with the provisions of the Delaware Business Trust Act. At the time trust preferred securities are to be issued, the original trust agreement for each trust will be amended and restated substantially in the form filed as an exhibit to the registration statement of which this prospectus is a part, to be effective at the time of such issuance. The amended and restated trust agreement, which we will refer to in this prospectus as the "trust agreement", will be qualified as an indenture under the Trust Indenture Act of 1939. Each trust will exist for the exclusive purposes of:


        Our debt securities will be the sole assets of each trust, and our payments under the debt securities will be the sole revenue of each trust. No separate financial statements of either trust will be included in this prospectus. We consider that these financial statements would not be material to holders of the trust preferred securities because the trusts would not have any independent operations and the only purposes of each trust are those described above. We do not expect that either trust will be filing annual, quarterly or special reports with the SEC. The principal place of business of each trust will be c/o Ameren Corporation, 1901 Chouteau Avenue, St. Louis, Missouri 63103.

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WHERE YOU CAN FIND MORE INFORMATION

        We and the trusts have filed a registration statement on Form S-3 with the SEC under the Securities Act of 1933. This prospectus is part of the registration statement, but the registration statement also contains or incorporates by reference additional information and exhibits. We are subject to the informational requirements of the Securities Exchange Act of 1934 and, therefore, we file annual, quarterly and current reports, proxy statements and other information with the SEC. You may read and copy the registration statement and any document that we file with the SEC at the SEC's public reference room at 450 Fifth Street, N.W., Washington, D.C. 20549. You can call the SEC's toll-free telephone number at 1-800-SEC-0330 for further information on the public reference room. The SEC maintains a web site at http://www.sec.gov that contains reports, proxy and information statements and other information regarding companies, such as us, that file documents with the SEC electronically. The documents can be found by searching the EDGAR Archives of the SEC electronically.

        The SEC allows us to "incorporate by reference" the information that we file with the SEC which means that we can disclose important information to you by referring you to those documents. The information incorporated by reference is considered to be part of this prospectus and you should read it with the same care. Later information that we file with the SEC will automatically update and supersede this information. We incorporate by reference the following documents previously filed with the SEC:

        We are also incorporating by reference all additional documents that we file with the SEC under Sections 13(a), 13(c), 14 and 15(d) of the Securities Exchange Act of 1934 between the date of this prospectus and the time that all of the securities registered are sold.

        You may request a free copy of these filings by writing or telephoning us at the following address:

        You should rely only on the information incorporated by reference or provided in this prospectus or any supplement. We have not authorized anyone else to provide you with different information. If anyone provides you with different or inconsistent information, you should not rely on it. We are not making an offer to sell these securities in any state where the offer or sale is not permitted. You should not assume that the information in this prospectus or any supplement is accurate as of any date other than the date on the front of those documents. Our business, financial condition, results of operations and prospects may have changed since those dates.

        The trusts will not be subject to the informational requirements of the Securities Exchange Act of 1934.

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RATIOS OF EARNINGS TO FIXED CHARGES

        Our ratio of earnings to fixed charges is computed by dividing our earnings by fixed charges before income taxes. For the purposes of such computations:


 
   Year Ended December 31,
   Three Months
Ended March 31,
 
   1997(1)
   1998
   1999
   2000
   2001
   2001
   2002
Ratio of Earnings to Fixed Charges   3.91   4.06   4.20   4.59   4.42   2.93   2.60
(1)
The ratio of earnings to fixed charges for the year ended December 31, 1997 reflects the accounting for the merger of CIPSCO Incorporated and Union Electric Company, which was completed on December 31, 1997, as a pooling of interests and is presented as if the two companies were combined as of the earliest period presented.


USE OF PROCEEDS

        Unless we state otherwise in any prospectus supplement, we may use the net proceeds we receive from any sale of the offered securities:

        The prospectus supplement relating to a particular offering of securities by us will identify the use of proceeds for that offering.

        The proceeds from the sale of trust preferred securities by a trust will be invested in debt securities issued by us. Except as we may otherwise describe in the related prospectus supplement, we expect to use the net proceeds from the sale of such debt securities to the applicable trust for the above purposes.

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FORWARD-LOOKING STATEMENTS

        Statements made in this prospectus, any accompanying prospectus supplement and the documents described under "Where You Can Find More Information," which are not based on historical facts, are "forward-looking" and, accordingly, involve risks and uncertainties that could cause actual results to differ materially from those discussed. Although such "forward-looking" statements have been made in good faith and are based on reasonable assumptions, there is no assurance that the expected results will be achieved. These statements include, without limitation, statements as to future expectations, beliefs, plans, strategies, objectives, events, conditions and financial performance. In connection with the "safe harbor" provisions of the Private Securities Litigation Reform Act of 1995, we are providing this cautionary statement to identify important factors that could cause actual results to differ materially from those anticipated. The following factors, in addition to those discussed elsewhere in this prospectus, any accompanying prospectus supplement and the documents described under "Where You Can Find More Information," could cause results to differ materially from management expectations as suggested by such "forward-looking" statements:

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        Given these uncertainties, you should not place undue reliance on these forward-looking statements. Except to the extent required by the federal securities laws, we undertake no obligation to publicly update or revise any forward-looking statements, whether as a result of new information, future events or otherwise.

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ABOUT THIS PROSPECTUS

        This prospectus is part of a registration statement that we filed with the SEC utilizing a "shelf" registration, or continuous offering, process. Under this shelf registration process, we or the trusts may issue and sell any combination of the securities described in this prospectus in one or more offerings with a maximum aggregate offering price of up to $1,472,875,000. We and the trusts may offer any of the following securities: senior debt securities or subordinated debt securities, each of which may be convertible into our common stock, trust preferred securities and related guarantees, common stock, stock purchase contracts and stock purchase units. We may also offer warrants to purchase debt securities or shares of our common stock.

        This prospectus provides you with a general description of the securities we and the trusts may offer. Each time we or the trusts sell securities, we or the applicable trust will provide a prospectus supplement that will contain specific information about the terms of that offering. Any prospectus supplement may also add, update or change information contained in this prospectus. If there is any inconsistency between the information in this prospectus and the prospectus supplement, you should rely on the information in the prospectus supplement. The registration statement we and the trusts filed with the SEC includes exhibits that provide more detail on descriptions of the matters discussed in this prospectus. You should read this prospectus and the related exhibits filed with the SEC and any prospectus supplement together with additional information described under the heading "Where You Can Find More Information."

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DESCRIPTION OF DEBT SECURITIES

General

        The senior debt securities and the subordinated debt securities, which we refer to collectively as the debt securities, will represent unsecured obligations of Ameren Corporation. We may issue one or more series of debt securities directly to the public, to a trust or as part of a stock purchase unit from time to time. We expect that each series of senior debt securities or subordinated debt securities will be issued as a new series of debt securities under one of two separate indentures, as each may be amended or supplemented from time to time. We will issue the senior debt securities in one or more series under a senior indenture dated as of December 1, 2001 between us and The Bank of New York, as trustee. We will issue the subordinated debt securities in one or more series under a subordinated indenture between us and a trustee. The senior indenture, the form of the subordinated indenture and the form of supplemental indenture or other instrument establishing the debt securities of a particular series are filed as exhibits to, or will be subsequently incorporated by reference in, the registration statement of which this prospectus is a part. Each indenture will be qualified under the Trust Indenture Act of 1939. The following summaries of certain provisions of the senior indenture, the subordinated indenture and the applicable debt securities do not purport to be complete and are subject to, and qualified in their entirety by, all of the provisions of the senior indenture or the subordinated indenture, as the case may be, and the applicable debt securities. We may also sell hybrid or novel securities now existing or developed in the future that combine certain features of the debt securities and other securities described in this prospectus. We may be required to obtain the approval of the SEC under the Public Utility Holding Company Act of 1935 before we can issue and sell certain of these securities.

        There is no requirement under the senior indenture, nor will there be any such requirement under, the subordinated indenture that our future issuances of debt securities be issued exclusively under either indenture, and we will be free to employ other indentures or documentation, containing provisions different from those included in either indenture or applicable to one or more issuances of senior debt securities or subordinated debt securities, as the case may be, in connection with future issuances of other debt securities. Nonetheless, the debt securities registered under the registration statement of which this prospectus is a part will only be issued pursuant to an indenture (or a form thereof) that is filed as an exhibit to the registration statement.

        The senior indenture provides and the subordinated indenture will provide that the applicable debt securities will be issued in one or more series, may be issued at various times, may have differing maturity dates and may bear interest at differing rates. We need not issue all debt securities of one series at the same time and, unless otherwise provided, we may reopen a series, without the consent of the holders of the senior debt securities or the subordinated debt securities of that series, as the case may be, for issuances of additional senior debt securities or subordinated debt securities of that series, as applicable. Unless otherwise described in the applicable prospectus supplement, neither indenture described above limits or will limit the aggregate amount of debt, including secured debt, we or our subsidiaries may incur.

Ranking

        The senior debt securities will be our direct unsecured general obligations and will rank equally with all of our other unsecured and unsubordinated debt. As of March 31, 2002, our aggregate outstanding debt that would have ranked equally with the senior debt securities was approximately $663 million, including $595 million of senior debt securities outstanding under the senior indenture. In addition, we have been granted authority by the SEC to issue up to $1.5 billion of guarantees for the benefit of our non-utility subsidiaries and expect to have such guarantees outstanding from time to time in various aggregate amounts. The subordinated debt securities will be our direct unsecured general

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obligations and will be junior in right of payment to our Senior Indebtedness, as described under the heading "—Subordination of Subordinated Debt Securities."

        Ameren is a holding company that derives substantially all of its income from its operating subsidiaries. As a result, our cash flows and consequent ability to service our debt, including the debt securities, are dependent upon the earnings of our subsidiaries and distribution of those earnings to us and other payments or distributions of funds by our subsidiaries to us, including payments of principal and interest under intercompany indebtedness. Our operating subsidiaries are separate and distinct legal entities and will have no obligation, contingent or otherwise, to pay any dividends or make any other distributions (except for payments required pursuant to the terms of intercompany indebtedness) to us or to otherwise pay amounts due with respect to the debt securities or to make specific funds available for such payments. Various financing arrangements, charter provisions and regulatory requirements may impose certain restrictions on the ability of our subsidiaries to transfer funds to us in the form of cash dividends, loans or advances. Furthermore, except to the extent we have a priority or equal claim against our subsidiaries as a creditor, the debt securities will be effectively subordinated to debt and preferred stock at the subsidiary level because, as the common shareholder of our subsidiaries, we will be subject to the prior claims of creditors of our subsidiaries. As of March 31, 2002, our subsidiaries had approximately $3.1 billion of aggregate outstanding debt and preferred stock.

Provisions of a Particular Series

        The prospectus supplement applicable to each issuance of debt securities will specify, among other things:

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        Unless otherwise indicated in the applicable prospectus supplement, there will be no provisions in either indenture or the related debt securities that require us to redeem, or permit the holders to cause a redemption of, those debt securities or that otherwise protect the holders in the event that we incur substantial additional indebtedness, whether or not in connection with a change in control, or grant security for other of our indebtedness.

Subordination of Subordinated Debt Securities

        The subordinated debt securities will be subordinate and junior in right of payment to all of our Senior Indebtedness, as defined below.

        No payment of principal of (including redemption and sinking fund payments), premium, if any, or interest on, the subordinated debt securities may be made if any Senior Indebtedness is not paid when due, any applicable grace period with respect to such default has ended and such default has not been cured or waived, or the maturity of any Senior Indebtedness has been accelerated because of a default and such acceleration has not been rescinded or annulled. If provided in the applicable prospectus supplement, limited subordination periods may apply in the event of non-payment defaults relating to Senior Indebtedness in situations where there has not been an acceleration of Senior Indebtedness.

        Upon any distribution of our assets to creditors upon any dissolution, winding-up, liquidation or reorganization, whether voluntary or involuntary, or in bankruptcy, insolvency, receivership or other proceedings, all principal of, and premium, if any, and interest due or to become due on, all Senior Indebtedness must be paid in full before the holders of the subordinated debt securities are entitled to receive or retain any payment. The rights of the holders of the subordinated debt securities will be subrogated to the rights of the holders of Senior Indebtedness to receive payments or distributions applicable to Senior Indebtedness until all amounts owing on the subordinated debt securities are paid in full.

        As defined in the subordinated indenture, the term "Senior Indebtedness" means:

        The subordinated indenture will not limit the aggregate amount of Senior Indebtedness that we may issue. As of March 31, 2002, our outstanding Senior Indebtedness aggregated approximately $663 million.

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Registration, Transfer and Exchange

        Unless otherwise indicated in the applicable prospectus supplement, each series of debt securities, other than debt securities issued to a trust, will initially be issued in the form of one or more global securities, in registered form, without coupons, as described under "Book-Entry System." The global securities will be registered in the name of The Depository Trust Company, as depository, or its nominee, and deposited with, or on behalf of, the depository. Except in the circumstances described under "Book-Entry System," owners of beneficial interests in a global security will not be entitled to have debt securities registered in their names, will not receive or be entitled to receive physical delivery of any debt securities and will not be considered the registered holders thereof under the applicable indenture.

        Debt securities of any series will be exchangeable for other debt securities of the same series of any authorized denominations and of a like aggregate principal amount and tenor. Subject to the terms of the applicable indenture and the limitations applicable to global securities, debt securities may be presented for exchange or registration of transfer—duly endorsed or accompanied by a duly executed instrument of transfer—at the office of any transfer agent we may designate for such purpose, without service charge but upon payment of any taxes and other governmental charges as described in the applicable indenture.

        Unless otherwise indicated in the applicable prospectus supplement, the transfer agent will be the trustee under the applicable indenture. We may at any time designate additional transfer agents or rescind the designation of any transfer agent or approve a change in the office through which any transfer agent acts, except that we will be required to maintain a transfer agent in each place of payment for the debt securities of each series.

Payment and Paying Agents

        Principal of and interest and premium, if any, on debt securities issued in the form of global securities will be paid in the manner described under "Book-Entry System."

        Unless otherwise indicated in the applicable prospectus supplement, the principal of and any premium and interest on debt securities of a particular series in the form of certificated securities will be payable at the office of the applicable trustee or at the authorized office of any paying agent or paying agents upon presentation and surrender of such debt securities. We may at any time designate additional paying agents or rescind the designation of any paying agent or approve a change in the office through which any paying agent acts, except that we will be required to maintain a paying agent in each place of payment for the debt securities of a particular series. Unless otherwise indicated in the applicable prospectus supplement, interest on the debt securities of a particular series, other than interest at maturity, that are in the form of certificated securities will be paid by check payable in clearinghouse funds mailed to the person entitled thereto at such person's address as it appears on the register for such debt securities maintained by the applicable trustee; provided, however, a holder of certificated securities in the aggregate principal amount of $10,000,000 or more will be entitled to receive payments of interest by wire transfer of immediately available funds to a bank within the continental United States if the trustee has received appropriate wire transfer instructions on or prior to the applicable regular record date for such interest payment date.

        All monies we pay to a trustee or a paying agent for the payment of the principal of, and premium or interest, if any, on, any debt security which remain unclaimed at the end of two years after such principal, premium or interest shall have become due and payable will be repaid to us, and the holder of such debt security thereafter may look only to us for payment thereof.

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Redemption

        Any terms for the optional or mandatory redemption of the debt securities will be set forth in the applicable prospectus supplement. Unless otherwise indicated in the applicable prospectus supplement, debt securities will be redeemable by us only upon notice by mail not less than 30 nor more than 60 days prior to the date fixed for redemption, and, if less than all the debt securities of a series are to be redeemed, the particular debt securities to be redeemed will be selected by such method as shall be provided for any particular series, or in the absence of any such provision, by the trustee in such manner as it shall deem fair and appropriate.

        Any notice of redemption at our option may state that such redemption will be conditional upon receipt by the trustee or the paying agent or agents, on or prior to the dated fixed for such redemption, of money sufficient to pay the principal of and premium, if any, and interest on, such debt securities and that if such money has not been so received, such notice will be of no force and effect and we will not be required to redeem such debt securities.

Events of Default

        Each of the following will constitute an event of default under the senior indenture or the subordinated indenture with respect to senior debt securities or subordinated debt securities, as the case may be, of any series:

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        No event of default with respect to the debt securities of a particular series necessarily constitutes an event of default with respect to the debt securities of any other series issued under the applicable indenture. If provided in the applicable prospectus supplement, an event of default similar to the event of default described in the fourth bullet above may be applicable to a series of subordinated debt securities.

        If an event of default with respect to any series of debt securities occurs and is continuing, then either the trustee for such series or the holders of a majority in aggregate principal amount of the outstanding debt securities of such series, by notice in writing, may declare the principal amount of and interest on all of the debt securities of such series to be due and payable immediately; provided, however, that if an event of default occurs and is continuing with respect to more than one series of debt securities under a particular indenture, the trustee for such series or the holders of a majority in aggregate principal amount of the outstanding debt securities of all such series, considered as one class, may make such declaration of acceleration and not the holders of the debt securities of any one of such series.

        At any time after an acceleration with respect to the debt securities of any series has been declared, but before a judgment or decree for the payment of the money due has been obtained, the event or events of default giving rise to such acceleration will be waived, and the acceleration will be rescinded and annulled, if

        However, no such waiver or rescission and annulment shall extend to or shall affect any subsequent default or impair any related right.

        Subject to the provisions of the applicable indenture relating to the duties of the trustee in case an event of default shall occur and be continuing, the trustee generally will be under no obligation to exercise any of its rights or powers under the applicable indenture at the request or direction of any of the holders unless such holders have offered to the trustee reasonable security or indemnity. Subject to such provisions for the indemnification of the trustee and certain other limitations contained in the applicable indenture, the holders of a majority in aggregate principal amount of the outstanding debt securities of any series will have the right to direct the time, method and place of conducting any proceeding for any remedy available to the trustee, or of exercising any trust or power conferred on the trustee, with respect to the debt securities of that series; provided, however, that if an event of default occurs and is continuing with respect to more than one series of debt securities, the holders of a majority in aggregate principal amount of the outstanding debt securities of all those series, considered as one class, will have the right to make such direction, and not the holders of the debt securities of any one series. Any direction provided by the holders shall not be in conflict with any rule of law or with the senior indenture or the subordinated indenture, as the case may be, and will not involve the

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trustee in personal liability in circumstances where reasonable indemnity would not, in the trustee's sole discretion, be adequate and the trustee may take any other action it deems proper that is not inconsistent with such direction.

        The holders of a majority in aggregate principal amount of the outstanding debt securities of any series may waive any past default under the applicable indenture on behalf of all holders of debt securities of that series with respect to the debt securities of that series, except a default in the payment of principal of or any premium or interest on such debt securities. No holder of debt securities of any series may institute any proceeding with respect to the applicable indenture, or for the appointment of a receiver or a trustee, or for any other remedy, unless such holder has previously given to the trustee for such series written notice of a continuing event of default with respect to the debt securities of such series, the holders of a majority in aggregate principal amount of the outstanding debt securities of all series in respect of which an event of default has occurred and is continuing, considered as one class, have made written request to the trustee for such series to institute such proceeding and have offered reasonable indemnity, and the trustee for such series has failed to institute such proceeding within 60 days after such notice, request and offer. Furthermore, no holder of debt securities of any series will be entitled to institute any such action if and to the extent that such action would disturb or prejudice the rights of other holders of those debt securities.

        Notwithstanding the foregoing, each holder of debt securities of any series has the right, which is absolute and unconditional, to receive payment of the principal of and premium and interest, if any, on such debt securities when due and to institute suit for the enforcement of any such payment, and such rights may not be impaired without the consent of that holder of debt securities.

        The trustee, within 90 days after it receives notice of the occurrence of a default with respect to the debt securities of any series, is required to give the holders of the debt securities of that series notice of such default, unless cured or waived, but, except in the case of default in the payment of principal of, or premium, if any, or interest on, the debt securities of that series, the trustee may withhold such notice if it determines in good faith that it is in the interest of such holders to do so. We will be required to deliver to the trustees for the debt securities each year a certificate as to whether or not, to the knowledge of the officers signing such certificate, we are in compliance with all conditions and covenants under the applicable indenture, determined without regard to any period of grace or requirement of notice under such indenture.

Modification

        Without the consent of any holder of debt securities, the trustee for such debt securities and we may enter into one or more supplemental indentures for any of the following purposes:

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        Except as provided above, the consent of the holders of a majority in aggregate principal amount of either the senior debt securities or the subordinated debt securities, as the case may be, of all series then outstanding, considered as one class, is required for the purpose of adding any provisions to, or changing in any manner, or eliminating any of the provisions of, the applicable indenture pursuant to one or more supplemental indentures or of modifying or waiving in any manner the rights of the holders of the applicable debt securities; provided, however, that if less than all of the series of senior debt securities or subordinated debt securities outstanding, as the case may be, are directly affected by a proposed supplemental indenture, then the consent only of the holders of a majority in aggregate principal amount of the outstanding applicable debt securities of all series so directly affected, considered as one class, will be required.

        Notwithstanding the foregoing, no such amendment or modification may, without the consent of each holder of outstanding debt securities affected thereby:

        A supplemental indenture which changes or eliminates any covenant or other provision of the applicable indenture which has expressly been included solely for the benefit of one or more series of debt securities, or which modifies the rights of the holders of debt securities of such series with respect to such covenant or provision, will be deemed not to affect the rights under the applicable indenture of the holders of the debt securities of any other series.

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Defeasance and Discharge

        Unless the applicable prospectus supplement states otherwise, we may elect either:

        In the case of either (1) or (2), we are required to deposit, in trust, with the applicable trustee money or U.S. government obligations, which through the payment of interest on those obligations and principal of those obligations in accordance with their terms will provide money, in an amount sufficient, without reinvestment, to pay all the principal of, premium, if any, and interest on the debt securities of such series on the dates payments are due (which may include one or more redemption dates designated by us). This trust may only be established if, among other things, (A) no event of default or event which with the giving of notice or lapse of time, or both, would become an event of default under the applicable indenture has occurred and is continuing on the date of the deposit, (B) the deposit will not cause the trustee to have any conflicting interest with respect to our other securities and (C) we have delivered an opinion of counsel to the effect that the holders will not recognize income, gain or loss for federal income tax purposes (and, in the case of paragraph (1) above, such opinion of counsel is based on a ruling of the Internal Revenue Service or other change in applicable federal income tax law) as a result of the deposit or defeasance and will be subject to federal income tax in the same amounts, in the same manner and at the same times as if the deposit and defeasance had not occurred.

        We may exercise our defeasance option under paragraph (1) with respect to debt securities of any series notwithstanding our prior exercise of our covenant defeasance option under paragraph (2). If we exercise our defeasance option for debt securities of any series, payment of the debt securities of such series may not be accelerated because of a subsequent event of default. If we exercise our covenant defeasance option for debt securities of any series, payment of the debt securities of such series may not be accelerated by reference to a subsequent breach of any of the covenants noted under clause (2) in the preceding paragraph. In the event we omit to comply with our remaining obligations with respect to the debt securities of any series under the applicable indenture after exercising our covenant defeasance option and the debt securities of such series are declared due and payable because of the subsequent occurrence of any event of default, the amount of money and U.S. government obligations on deposit with the trustee may be insufficient to pay amounts due on the debt securities of such series at the time of the acceleration resulting from that event of default. However, we will remain liable for those payments.

Consolidation, Merger and Sale or Disposition of Assets

        We have agreed not to consolidate with or merge into any other corporation or sell or otherwise dispose of our properties as or substantially as an entirety to any person, unless:

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        Upon any such consolidation, merger, sale, transfer or other disposition of our properties as or substantially as an entirety, the successor corporation formed by such consolidation or into which we are merged or the person to which such sale, transfer or other disposition is made shall succeed to, and be substituted for, and may exercise every right and power of, us under the applicable indenture with the same effect as if such successor corporation or person had been named as us therein, and we will be released from all obligations under the applicable indenture.

        Certain of the indentures for debt securities issued or to be issued by AmerenUE and AmerenCIPS provide that the conveyance or other transfer by either company of all or any portion of its facilities for the generation of electric energy, all of its facilities for the transmission of electric energy, or all of its facilities for the distribution of natural gas, in each case considered alone or in any combination with properties described in any other clause, shall in no event be deemed to constitute a conveyance or other transfer of all of its properties, as or substantially as an entirety.

Resignation or Removal of Trustee

        The trustee may resign at any time upon written notice to us specifying the day upon which the resignation is to take effect and such resignation will take effect immediately upon the later of the appointment of a successor trustee and such specified day. The trustee may be removed at any time with respect to debt securities of any series by an instrument or concurrent instruments in writing filed with the trustee and signed by the holders, or their attorneys-in-fact, of a majority in aggregate principal amount of that series of debt securities then outstanding. In addition, so long as no event of default or event which, with the giving of notice or lapse of time or both, would become an event of default has occurred and is continuing, we may remove the trustee upon notice to the holder of each debt security outstanding and the trustee, and appointment of a successor trustee.

Concerning the Trustee for Senior Debt Securities

        We and our affiliates maintain corporate trust and other normal banking relationships with The Bank of New York, the trustee under the senior indenture. Each indenture provides that our obligations to compensate the trustee and reimburse the trustee for expenses, disbursements and advances will be secured by a lien prior to that of the applicable debt securities upon the property and funds held or collected by the trustee as such.

Governing Law

        Each of the senior indenture and the subordinated indenture and the related debt securities will be governed by New York law.

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DESCRIPTION OF TRUST PREFERRED SECURITIES

        This prospectus describes certain general terms of the trust preferred securities. The trust preferred securities will be issued by Ameren Capital Trust I or Ameren Capital Trust II, each a Delaware business trust which we formed by executing a trust agreement for each trust (with The Bank of New York (Delaware), as trustee, and an administrator named therein) and causing the filing of a certificate of trust for each trust with the Delaware Secretary of State. At the time trust preferred securities are to be issued by a trust, the original trust agreement for such trust will be amended and restated, to be effective at the time of such issuance. The form of amended and restated trust agreement is filed as an exhibit to the registration statement of which this prospectus is a part. The amended and restated trust agreement, which we will refer to in this prospectus as the "trust agreement," for each trust will be qualified as an indenture under the Trust Indenture Act of 1939. You should read the form of amended and restated trust agreement for provisions that may be important to you. When we offer to sell a particular series of trust preferred securities, we will describe the specific terms of that series in a prospectus supplement. The trust preferred securities will be issued pursuant to the related trust agreement, which we have summarized below. This summary is not complete.

General

        Each trust will exist for the exclusive purposes of:

        Our debt securities will be the sole assets of each trust, and our payments under the debt securities will be the sole revenue of each trust. No separate financial statements of either trust will be included in this prospectus. We consider that these financial statements would not be material to holders of the trust preferred securities because the trusts would not have any independent operations and the only purposes of each trust are those described above. We do not expect that either trust will be filing annual, quarterly or special reports with the SEC. The principal place of business of each trust will be c/o Ameren Corporation, 1901 Chouteau Avenue, St. Louis, Missouri 63103.

        Each trust will exist until terminated as provided in its trust agreement. The administrators and trustees of each trust will be:

        The trust agreement will authorize the administrators to issue two classes of trust securities: trust preferred securities and trust common securities. We will own all of the trust common securities issued by each trust, which will rank equally in right of payment with the trust preferred securities issued by the respective trust. However, if an event of default occurs and is continuing under the trust agreement, rights of the holders of the trust common securities to payment for distributions and otherwise will be subordinated to the rights of the holders of the trust preferred securities. We will acquire trust common

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securities of each trust in a total liquidation amount of at least three percent of the total capital of the trust.

        Proceeds from the sale of both the trust preferred securities and the trust common securities issued by each trust will be used to purchase our debt securities, which will be held in trust by the property trustee for the benefit of the holders of the trust securities issued by the respective trust. We will guarantee the payments of distributions and payments of redemption or liquidation with respect to the trust preferred securities issued by each trust, but only to the extent the respective trust has funds available to make those payments and has not made the payments. See "Description of Guarantees" below.

        Each guarantee, when taken together with our obligations under the related debt securities, the related indenture and the related trust agreement (including our agreement to pay the expenses of the trust, other than payments in respect of the trust securities, contained in the trust agreement), will provide a full and unconditional guarantee of amounts due on the trust preferred securities issued by the respective trust. The trust preferred securities will have the terms, including distributions, redemption, voting, liquidation rights and other rights or restrictions that will be described in the related trust agreement or made part of it by the Trust Indenture Act or the Delaware Business Trust Act.

Provisions of a Particular Series

        Each trust may issue only one series of trust preferred securities. The applicable prospectus supplement will set forth the principal terms of the trust preferred securities that will be offered, including:

        Terms of the trust preferred securities issued by each trust will mirror the terms of the debt securities held by the respective trust. In other words, the interest rate and interest and other payment dates of each series of debt securities issued to a trust will correspond to the rate at which distributions will be paid and the distribution and other payment dates of the trust preferred securities of that trust. The prospectus supplement will also set forth whether the debt securities to be issued to a trust will be senior debt securities or subordinated debt securities.

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Distributions

        The trust preferred securities represent preferred, undivided, beneficial interests in the assets of the respective trust. The applicable prospectus supplement will state the annual rate, as a percentage of the liquidation amount, at which distributions on each trust preferred security will be payable, the liquidation amount and the dates on which distributions will be payable.

        Each trust will use the proceeds from the issuance and sale of the trust preferred securities to purchase debt securities. The revenue of a trust available for distribution to holders of the trust preferred securities issued by that trust will be limited to payments under those debt securities. If we do not make payments on the debt securities, a trust will not have funds available to pay distributions or other amounts payable on the trust preferred securities issued by that trust. The payment of distributions and other amounts payable on the trust preferred securities issued by a trust, if and to the extent the trust has funds legally available for and cash sufficient to make such payments, is guaranteed by us on a limited basis.

Option to Extend Interest Payment Period

        Unless the applicable prospectus supplement states otherwise, we have the right to defer the payment of interest on the debt securities at any time or from time to time for a period, which we refer to in this prospectus as an "extension period," not exceeding 20 consecutive quarterly periods with respect to each extension period. During each extension period we shall have the right to make partial payments of interest on any interest payment date. At the end of each extension period we shall pay all interest then accrued and unpaid. No extension period may extend beyond the stated maturity of the debt securities or end on a date other than an interest payment date. As a consequence of any such deferral, distributions on the trust preferred securities by a trust will be deferred during any such extension period. Distributions to which holders of the trust preferred securities are entitled will accumulate additional distributions at the rate stated in the applicable prospectus supplement. During an extension period, interest will continue to accrue and holders of debt securities, or holders of trust preferred securities while outstanding, will be required to accrue original issue discount income for United States federal income tax purposes.

        Prior to the termination of any extension period, we may further defer the payment of interest, provided that, unless the applicable prospectus supplement states otherwise, no extension period may exceed 20 consecutive quarterly periods or extend beyond the stated maturity of the debt securities. Upon the termination of any extension period and the payment of all amounts then due, we may elect to begin a new extension period subject to the above conditions. No interest shall be due and payable during an extension period, except at its end. We must give the trustee notice of our election of an extension period at least one business day prior to the earlier of the date the distributions on the trust preferred securities would have been payable but for the election to begin such extension period and the date the property trustee is required to give notice to holders of the trust preferred securities of the record date or the date such distributions are payable, but in any event not less than one business day prior to such record date. The trustee will give notice of our election to begin a new extension period to the holders of the trust preferred securities.

Registration, Transfer and Exchange

        Unless otherwise indicated in the applicable prospectus supplement, each series of trust preferred securities will be issued initially in the form of one or more global securities, in registered form, without coupons, as described under "Book-Entry System." The global trust preferred securities will be registered in the name of a nominee of The Depository Trust Company, as depository, and deposited with, or on behalf of, the depository. Except in the circumstances described under "Book-Entry System," owners of beneficial interests in a global trust preferred security will not be entitled to have

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trust preferred securities registered in their names, will not receive or be entitled to receive physical delivery of any trust preferred securities and will not be considered the registered holders thereof under the related trust agreement.

        Trust preferred securities of any series will be exchangeable for other trust preferred securities of the same series of any authorized denominations of a like aggregate liquidation amount and tenor. Subject to the terms of the trust agreement and the limitations applicable to global securities, trust preferred securities may be presented for exchange or registration of transfer—duly endorsed or accompanied by a duly executed instrument of transfer—at the office of the property trustee, without service charges but upon payment of any taxes and other governmental charges as described in the trust agreement. Such transfer or exchange will be effected upon the property trustee being satisfied with the documents of title and identity of the person making the request.

        The property trustee will not be required to issue, register the transfer of, or exchange any trust preferred securities during a period beginning at the opening of business 15 days before the day of mailing of a notice of redemption of any trust preferred securities called for redemption and ending at the close of business on the day of mailing or register the transfer of, or exchange, any trust preferred securities selected for redemption except, in the case of any trust preferred security to be redeemed in part, the portion thereof not to be so redeemed.

Payment and Paying Agents

        Distributions and other payments on trust preferred securities issued in the form of global securities will be paid in the manner described under "Book-Entry System."

        Unless otherwise indicated in the applicable prospectus supplement, distributions and other payments with respect to trust preferred securities that are in the form of certificated securities will be made by check mailed to the person entitled thereto at such person's address as such address appears on the securities register for the trust securities maintained by the property trustee. The paying agent initially will be the property trustee and any co-paying agent chosen by the property trustee and acceptable to the administrators. If the property trustee is no longer the paying agent, the property trustee will appoint a successor, which must be a bank or trust company reasonably acceptable to the administrators, to act as paying agent. Such paying agent will be permitted to resign as paying agent upon 30 days' written notice to the property trustee and the administrators at which time the paying agent will return all unclaimed funds and all other funds in its possession to the property trustee.

Redemption

        Upon the repayment or redemption, in whole or in part, of the debt securities held by a trust, the proceeds shall be applied by the property trustee to redeem a Like Amount, as defined below, of the trust securities issued by that trust, upon not less than 30 nor more than 60 days' notice, unless otherwise indicated in a prospectus supplement, at a redemption price equal to the aggregate liquidation amount of the trust preferred securities plus accumulated but unpaid distributions to but excluding the redemption date and the related amount of the premium, if any, paid by us upon the concurrent redemption of the debt securities. If less than all the debt securities held by a trust are to be repaid or redeemed on a redemption date, then the proceeds from the repayment or redemption shall be allocated to the redemption proportionately of the trust preferred securities and the trust common securities issued by that trust based on the relative liquidation amounts of the classes. The amount of premium, if any, paid by us upon the redemption of all or any part of the debt securities held by a trust to be repaid or redeemed on a redemption date shall be allocated to the redemption proportionately of the trust preferred securities and the trust common securities issued by that trust.

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        Unless the applicable prospectus supplement states otherwise, we will have the right to redeem the debt securities held by a trust:

        "Like Amount" means:

        "Tax Event" means the receipt by a trust of an opinion of counsel to us experienced in relevant matters to the effect that, as a result of any amendment to, or change—including any announced prospective change—in, the laws or any regulations thereunder of the United States or any political subdivision or taxing authority of or in the United States, or as a result of any official or administrative pronouncement or action or judicial decision interpreting or applying these laws or regulations, which amendment or change is effective or which pronouncement or decision is announced on or after the date of issuance by a trust of trust preferred securities, including, without limitation, any of the foregoing arising with respect to, or resulting from, any proposal, proceeding or other action commencing on or before the date of issuance, there is more than an insubstantial risk that:

        "Investment Company Event" means the receipt by a trust of an opinion of counsel to us experienced in these matters to the effect that, as a result of the occurrence of a change in law or regulation or a written change—including any announced prospective change—in interpretation or application of law or regulation by any legislative body, court, governmental agency or regulatory authority, there is more than an insubstantial risk that the trust is or will be considered an "investment company" that is required to be registered under the Investment Company Act, which change or prospective change becomes effective or would become effective, as the case may be, on or after the date of the issuance by that trust of trust preferred securities.

        If and for so long as a trust is the holder of all the debt securities issued by us to that trust, we will pay, with respect to the debt securities, such additional amounts as may be necessary in order that the amount of distributions then due and payable by a trust on the outstanding trust preferred

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securities and trust common securities of a trust will not be reduced as a result of any additional taxes, duties and other governmental charges to which that trust has become subject, including as a result of a Tax Event.

Redemption Procedures

        Trust preferred securities of a trust redeemed on each redemption date shall be redeemed at the redemption price with the applicable proceeds from the contemporaneous redemption of the debt securities held by that trust. Redemptions of trust preferred securities shall be made and the redemption price shall be payable on each redemption date only to the extent that a trust has funds on hand available for the payment of the redemption price. See also "—Subordination of Trust Common Securities."

        If a trust gives a notice of redemption in respect of any trust preferred securities, then, by 12:00 noon, New York City time, on the redemption date, to the extent funds are available, in the case of trust preferred securities held in book-entry form, the property trustee will deposit irrevocably with the depository funds sufficient to pay the applicable redemption price and will give the depository irrevocable instructions and authority to pay the redemption price to the holders of the trust preferred securities. With respect to trust preferred securities not held in book-entry form, the property trustee, to the extent funds are available, will irrevocably deposit with the paying agent for the trust preferred securities funds sufficient to pay the applicable redemption price and will give the paying agent irrevocable instructions and authority to pay the redemption price to the holders upon surrender of their certificates evidencing the trust preferred securities. Notwithstanding the foregoing, distributions payable on or prior to the redemption date for any trust preferred securities called for redemption shall be payable to the holders of the trust preferred securities on the relevant record dates for the related distribution dates. If notice of redemption shall have been given and funds deposited as required, then upon the date of the deposit all rights of the holders of the trust preferred securities so called for redemption will cease, except the right of the holders of the trust preferred securities to receive the redemption price, and any distribution payable in respect of the trust preferred securities, but without interest on the redemption price, and the trust preferred securities will cease to be outstanding. In the event that payment of the redemption price in respect of trust preferred securities called for redemption is improperly withheld or refused and not paid either by a trust or by us pursuant to the guarantee as described under "Description of Guarantees," distributions on the trust preferred securities will continue to accumulate at the then applicable rate, from the redemption date originally established by a trust for the trust preferred securities it issues to the date the redemption price is actually paid, in which case the actual payment date will be the date fixed for redemption for purposes of calculating the redemption price.

        If less than all the trust preferred securities and trust common securities are to be redeemed on a redemption date, then the aggregate liquidation amount of the trust preferred securities and trust common securities to be redeemed shall be allocated proportionately to the trust preferred securities and the trust common securities based upon the relative liquidation amounts of the classes. The particular trust preferred securities to be redeemed shall be selected on a proportionate basis not more than 60 days prior to the redemption date by the property trustee from the outstanding trust preferred securities not previously called for redemption, or if the trust preferred securities are then held in the form of a global trust preferred security, in accordance with the depository's customary procedures. The property trustee shall promptly notify the securities registrar for the trust securities in writing of the trust preferred securities selected for redemption and, in the case of any trust preferred securities selected for partial redemption, the liquidation amount to be redeemed. For all purposes of the trust agreements, unless the context otherwise requires, all provisions relating to the redemption of trust preferred securities shall relate, in the case of any trust preferred securities redeemed or to be

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redeemed only in part, to the portion of the aggregate liquidation amount of trust preferred securities which has been or is to be redeemed.

        Notice of any redemption will be mailed at least 30 days but not more than 60 days before the redemption date to each registered holder of trust preferred securities to be redeemed at its address appearing on the securities register for the trust securities. Unless we default in payment of the redemption price on the related debt securities, on and after the redemption date interest will cease to accrue on the debt securities or portions of them called for redemption.

Subordination of Trust Common Securities

        If on any distribution date or redemption date a payment event of default with respect to the underlying debt securities has occurred and is continuing, no payment on or in respect of the related trust common securities shall be made unless all amounts due in respect of the related trust preferred securities (including the liquidation amount or redemption price, if applicable) shall have been paid or payment provided for. All funds immediately available to the respective property trustee shall first be applied to the payment in full in cash of all distributions on, or redemption price of, the trust preferred securities then due and payable.

        In the case of any event of default, as defined below, resulting from an event of default with respect to the underlying debt securities, the holders of trust common securities will be deemed to have waived any right to act with respect to any event of default under the related trust agreement until the effects of all events of default with respect to the related trust preferred securities have been cured, waived or otherwise eliminated. See "—Events of Default" and "Description of Debt Securities—Events of Default." Until all events of default under the related trust agreement with respect to the trust preferred securities have been so cured, waived or otherwise eliminated, the property trustee will act solely on behalf of the holders of the trust preferred securities and not on behalf of the holders of the trust common securities, and only the holders of the trust preferred securities will have the right to direct the property trustee to act on their behalf.

Liquidation Distribution Upon Dissolution

        In the event of any liquidation of a trust, the applicable prospectus supplement will state the amount payable on the trust preferred securities issued by that trust as a dollar amount per trust preferred security plus accumulated and unpaid distributions to the date of payment, subject to certain exceptions, which may be in the form of a distribution of the amount in debt securities held by that trust.

        The holders of all the outstanding trust common securities of a trust have the right at any time to dissolve the trust and, after satisfaction of liabilities to creditors of the trust as provided by applicable law, cause the debt securities held by that trust to be distributed in liquidation of the trust to the holders of the trust preferred securities and trust common securities issued by the trust.

        Pursuant to the related trust agreement, unless the applicable prospectus supplement states otherwise, a trust will automatically dissolve upon expiration of its term or, if earlier, will dissolve on the first to occur of:

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        If dissolution of a trust occurs as described in the first, second or fourth bullet point above, the trust will be liquidated by the property trustee as expeditiously as the property trustee determines to be possible by distributing, after satisfaction of liabilities to creditors of the trust as provided by applicable law, to the holders of the trust securities issued by the trust a Like Amount of the related debt securities. If such distribution is not practical, or, if a dissolution of a trust occurs as described in the third bullet point above, the holders will be entitled to receive out of the assets of the trust available for distribution to holders, after satisfaction of liabilities to creditors of the trust as provided by applicable law, an amount equal to, in the case of holders of the trust preferred securities, the aggregate of the liquidation amount plus accumulated and unpaid distributions to the date of payment. In this prospectus we refer to this amount as the "liquidation distribution." If the liquidation distribution can be paid only in part because the trust has insufficient assets available to pay in full the aggregate liquidation distribution, then the amounts payable directly by the trust on its trust preferred securities shall be paid on a proportionate basis. The holders of the trust common securities issued by the trust will be entitled to receive distributions upon any liquidation proportionately with the holders of the trust preferred securities, except that if a payment event of default has occurred and is continuing on the related debt securities, the trust preferred securities shall have a priority over the trust common securities. See "—Subordination of Trust Common Securities."

        After the liquidation date is fixed for any distribution of debt securities we have issued to a trust,

        If we do not redeem the debt securities we have issued to a trust prior to the stated maturity and the trust is not liquidated and the debt securities are not distributed to holders of the trust preferred securities issued by that trust, the trust preferred securities will remain outstanding until the repayment of the debt securities and the distribution of the liquidation distribution to the holders of the trust preferred securities.

        There can be no assurance as to the market prices for trust preferred securities or the related debt securities that may be distributed in exchange for trust preferred securities if a dissolution and liquidation of a trust were to occur. Accordingly, the trust preferred securities that an investor may purchase, or the related debt securities that the investor may receive on dissolution and liquidation of a trust, may trade at a discount to the price that the investor paid to purchase the trust preferred securities offered hereby.

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Certain Covenants

        In connection with the issuance of trust preferred securities by a trust, we will agree:

        Unless the applicable prospectus supplement states otherwise, we will also agree that we will not, directly or indirectly (1) declare or pay any dividends or distributions on, or redeem, purchase, acquire or make a liquidation payment with respect to, any of our capital stock, (2) make any payment of principal of or interest or premium, if any, on or repay, repurchase or redeem any debt securities of ours that rank equally with, or junior to, the debt securities (or, with respect to senior deferrable debt securities, make any payment of interest on senior deferrable debt securities with similar deferral provisions or make any payment of principal of or interest or premium, if any, on or repay, repurchase or redeem any debt securities that rank junior to such senior deferrable debt securities), or (3) make any guarantee payments with respect to any guarantee issued by us if such guarantee ranks equally with, or junior to, the debt securities (or, with respect to senior deferrable debt securities, make any guarantee payments with respect to any guarantee issued by us if such guarantee ranks junior to such senior deferrable debt securities), other than, in each case:

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        if at such time

        We will also agree that, if and for so long as a trust is the holder of all debt securities issued by us in connection with the issuance of trust preferred securities by that trust and that trust is required to pay any additional taxes, duties or other governmental charges, including in connection with a Tax Event, we will pay as additional sums on the debt securities the amounts that may be required so that the distributions payable by that trust will not be reduced as a result of any additional taxes, duties or other governmental charges.

Events of Default

        Any one of the following events constitutes an event of default with respect to the trust preferred securities issued by a trust under the related trust agreement:


        Within five business days after the occurrence of certain events of default actually known to the respective property trustee, the property trustee will transmit notice of the event of default to the respective holders of trust securities and the respective administrators, unless the event of default has been cured or waived. Within five business days after the receipt of notice that we intend to exercise our right under the applicable indenture to defer the payment of interest on the related debt securities, the property trustee must notify the holders and the administrators that we intend to defer these interest payments, unless we have revoked our determination to do so.

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        The applicable trust agreement includes provisions as to the duties of the property trustee in case an event of default occurs and is continuing. Consistent with these provisions, the property trustee will be under no obligation to exercise any of its rights or powers at the request or direction of any of the holders unless those holders have offered to the property trustee reasonable indemnity. Subject to these provisions for indemnification, the holders of a majority in liquidation amount of the related outstanding trust preferred securities may direct the time, method and place of conducting any proceeding for any remedy available to the property trustee, or exercising any trust or power conferred on the property trustee, with respect to the related trust preferred securities.

        The holders of at least a majority in aggregate liquidation amount of the outstanding trust preferred securities issued by a trust may waive any past default under the applicable trust agreement except:

        If an event of default under the applicable indenture has occurred and is continuing as a result of any failure by us to pay any amounts when due in respect of the related debt securities issued by us to a trust, the related trust preferred securities will have a preference over the related trust common securities with respect to payments of any amounts in respect of the trust preferred securities as described above. See "—Subordination of Trust Common Securities," "—Liquidation Distribution Upon Dissolution" and "Description of Debt Securities—Events of Default."

        We must furnish annually to each property trustee a statement by an appropriate officer as to that officer's knowledge of our compliance with all conditions and covenants under the respective trust agreement. Also, the administrators for each trust must file, on behalf of the respective trust, a statement as to our compliance with all conditions and covenants under the respective trust agreement.

Voting Rights; Amendment of Trust Agreement

        Except as provided below and under "—Resignation, Removal of Property Trustee and Delaware Trustee; Appointment of Successors" and "Description of Guarantees—Amendments and Assignment" and as otherwise required by law and the applicable trust agreement, the holders of the trust preferred securities issued by a trust will have no voting rights.

        The trust agreement applicable to a trust may be amended from time to time by the holders of a majority in liquidation amount of its trust common securities and the respective property trustee, without the consent of the holders of the trust preferred securities issued by the trust:

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        The trust agreement applicable to a trust may be amended by the holders of a majority in aggregate liquidation amount of the trust common securities and the property trustee with the consent of holders representing not less than a majority in aggregate liquidation amount of the outstanding trust preferred securities and receipt by the property trustee and the Delaware trustee of an opinion of counsel to the effect that the amendment or the exercise of any power granted to the trustees in accordance with the amendment will not affect the trust's not being taxable other than as a grantor trust for United States federal income tax purposes or the trust's exemption from status as an "investment company" under the Investment Company Act.

        Without the consent of each holder of trust preferred securities affected by the amendment or related exercise of power, the trust agreement applicable to a trust may not be amended to change the amount or timing of any distribution on the trust securities or otherwise adversely affect the amount of any distribution required to be made in respect of the trust securities as of a specified date or restrict the right of a holder of trust securities to institute suit for the enforcement of any payment due.

        So long as any debt securities are held by a trust, the respective property trustee will not:

without, in each case, obtaining the prior approval of the holders of at least a majority in aggregate liquidation amount of the trust preferred securities issued by such trust, except that, if a consent under the applicable indenture would require the consent of each holder of debt securities affected by the consent, no consent will be given by the property trustee without the prior written consent of each holder of the trust preferred securities issued by such trust.

        A property trustee may not revoke any action previously authorized or approved by a vote of the holders of the trust preferred securities issued by its respective trust except by subsequent vote of the holders of such trust preferred securities. The property trustee will notify each holder of trust preferred securities of any notice of default with respect to the related debt securities. In addition, before taking any of the foregoing actions, the property trustee will obtain an opinion of counsel experienced in relevant matters to the effect that the trust will not be taxable other than as a grantor trust for United States federal income tax purposes on account of the action.

        Any required approval of holders of trust preferred securities issued by a trust may be given at a meeting of holders of those trust preferred securities convened for the purpose or pursuant to written consent. The property trustee will cause a notice of any meeting at which holders of trust preferred securities are entitled to vote, or of any matter upon which action by written consent of the holders is

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to be taken, to be given to each registered holder of trust preferred securities in the manner set forth in the applicable trust agreement.

        No vote or consent of the holders of trust preferred securities issued by a trust will be required to redeem and cancel those trust preferred securities in accordance with the applicable trust agreement. See above under "—Redemption."

        Notwithstanding that holders of trust preferred securities issued by a trust are entitled to vote or consent under any of the circumstances described above, any of those trust preferred securities that are owned by us, the respective property trustee or Delaware trustee, or any affiliate of us or either trustee, will, for purposes of the vote or consent, be treated as if they were not outstanding.

Enforcement of Certain Rights by Holders of Trust Preferred Securities

        If an event of default has occurred and is continuing under the applicable indenture, and the trustee for the related debt securities and the holders of those debt securities have failed to declare the principal due and payable, the holders of at least 33% in aggregate liquidation amount of the related outstanding trust preferred securities shall have this right.

        If an event of default has occurred and is continuing under a trust agreement and the event is attributable to our failure to pay any amounts payable in respect of debt securities on the date the amounts are otherwise payable, a registered holder of trust preferred securities may institute a direct action against us for enforcement of payment to the holder of an amount equal to the amount payable in respect of debt securities having a principal amount equal to the aggregate liquidation amount of the trust preferred securities held by the holder, which we refer to in this discussion as a "Direct Action." We will have the right under the applicable indenture to set-off any payment made to the holders of trust preferred securities by us in connection with a Direct Action.

        We may not amend the applicable indenture to remove the foregoing right to bring a Direct Action without the prior written consent of the holders of all the trust preferred securities. Furthermore, so long as any of the trust preferred securities are outstanding

without the prior consent of the holders of at least a majority of the aggregate liquidation amount of the outstanding trust preferred securities unless and until the principal of, accrued and unpaid interest on and premium, if any, on the related debt securities have been paid in full and certain other conditions are satisfied.

        With certain exceptions, the holders of the trust preferred securities would not be able to exercise directly any remedies available to the holders of the debt securities except under the circumstances described in this section.

Resignation, Removal of Property Trustee and Delaware Trustee; Appointment of Successors

        The property trustee or the Delaware trustee of a trust may resign at any time by giving written notice to us or may be removed at any time by an action of the holders of a majority in liquidation amount of that trust's outstanding trust preferred securities delivered to the trustee to be removed and to us. No resignation or removal of either of the trustees and no appointment of a successor trustee will become effective until a successor trustee accepts appointment in accordance with the requirements

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of the trust agreement. So long as no event of default or event that would become an event of default has occurred and is continuing, and except with respect to a trustee appointed by an action of the holders, if we have delivered to either the property trustee or the Delaware trustee a resolution of our board of directors appointing a successor trustee and the successor trustee has accepted the appointment in accordance with the terms of the trust agreement, the property trustee or the Delaware trustee, as the case may be, will be deemed to have resigned and the successor trustee will be deemed to have been appointed as trustee in accordance with the trust agreement.

Mergers, Consolidations, Amalgamations or Replacements of a Trust

        A trust may not merge with or into, consolidate, amalgamate, or be replaced by, or convey, transfer or lease its properties and assets substantially as an entirety to any entity, except as described below or as otherwise set forth in the applicable trust agreement. A trust may, at the request of the holders of its trust common securities and with the consent of the holders of at least a majority in aggregate liquidation amount of its outstanding trust preferred securities, merge with or into, consolidate, amalgamate, or be replaced by or convey, transfer or lease its properties and assets substantially as an entirety to a trust organized as such under the laws of any state, so long as:


        Notwithstanding the foregoing, a trust may not, except with the consent of holders of 100% in aggregate liquidation amount of its trust preferred securities, consolidate, amalgamate, merge with or

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into, or be replaced by or convey, transfer or lease its properties and assets substantially as an entirety to, any other entity or permit any other entity to consolidate, amalgamate, merge with or into, or replace it if the consolidation, amalgamation, merger, replacement, conveyance, transfer or lease would cause the trust or the successor entity to be taxable other than as a grantor trust for United States federal income tax purposes.

Expenses and Taxes

        In each of the trust agreements, we have agreed to pay:

Information Concerning the Property Trustees

        Each property trustee, other than during the occurrence and continuance of an event of default, undertakes to perform only the duties as are specifically set forth in the applicable trust agreement and, after an event of default, must exercise the same degree of care and skill as a prudent person would exercise or use in the conduct of his or her own affairs. Subject to this provision, each property trustee is under no obligation to exercise any of the powers vested in it by the trust agreements at the request of any holder of trust preferred securities issued by the respective trust unless it is offered reasonable indemnity against the costs, expenses and liabilities that might be incurred by exercising these powers.

Miscellaneous

        The administrators and the property trustee relating to each trust are authorized and directed to conduct the affairs of and to operate the trust in such a way that the trust will not be deemed to be an "investment company" required to be registered under the Investment Company Act or taxable other than as a grantor trust for United States federal income tax purposes and so that the debt securities held by that trust will be treated as indebtedness of ours for United States federal income tax purposes. In this regard, the administrators, each property trustee and the holders of trust common securities issued by the respective trust are authorized to take any action, not inconsistent with applicable law, the certificate of trust of the trust or the applicable trust agreement, that the administrators, the property trustee and the holders of trust common securities determine in their discretion to be necessary or desirable for these purposes, as long as this action does not materially adversely affect the interests of the holders of the trust preferred securities.

        Holders of the trust preferred securities have no preemptive or similar rights.

        A trust may not borrow money or issue debt or mortgage or pledge any of its assets.

Governing Law

        The trust agreement and the trust preferred securities of each trust will be governed by Delaware law.

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DESCRIPTION OF GUARANTEES

        Each guarantee will be executed and delivered by us concurrently with the issuance of trust preferred securities by a trust for the benefit of the holders from time to time of the trust preferred securities. We will appoint a guarantee trustee under each guarantee. Each guarantee trustee will hold the respective guarantee for the benefit of the holders of the trust preferred securities issued by the related trust. Each guarantee will be qualified as an indenture under the Trust Indenture Act of 1939. We have summarized below certain provisions of the guarantees. This summary does not purport to be complete and is subject to, and qualified in its entirety by reference to, all the provisions of the guarantee, including the definitions in the guarantee of certain terms. The form of guarantee agreement is filed as an exhibit to the registration statement of which this prospectus is a part.

General

        We will fully and unconditionally agree, to the extent described herein, to pay the guarantee payments, as defined below, to the holders of the trust preferred securities issued by each trust, as and when due, regardless of any defense, right of set-off or counterclaim that a trust may have or assert other than the defense of payment. The following payments with respect to the trust preferred securities, to the extent not paid or made by or on behalf of the respective trust, which payments we refer to in this discussion as the "guarantee payments," will be subject to the respective guarantee:

        Our obligation to make a guarantee payment may be satisfied by direct payment of the required amounts by us to the holders of the trust preferred securities or by causing the trust to pay these amounts to the holders.

        Each guarantee will be an irrevocable guarantee of the obligations of the respective trust under its trust preferred securities, but will apply only to the extent that the trust has funds sufficient to make these payments, and is not a guarantee of collection.

        If we do not make payments on the debt securities held by a trust, the trust will not be able to pay any amounts payable in respect of its trust preferred securities and will not have funds legally available for these payments. The applicable prospectus supplement will describe the ranking of the guarantee. See "—Status of the Guarantees." The guarantees do not limit our incurrence or issuance of other secured or unsecured debt, including Senior Indebtedness, whether under the applicable indenture, any other indenture that we may enter into in the future or otherwise.

        We have, through the guarantees, the trust agreements (including our agreement to pay the expenses of the trust contained in the related trust agreement), the applicable debt securities and the

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related indenture, taken together, fully, irrevocably and unconditionally guaranteed all of each trust's obligations under its trust preferred securities. No single document standing alone or operating in conjunction with fewer than all the other documents constitutes the guarantee. It is only the combined operation of these documents that has the effect of providing a full, irrevocable and unconditional guarantee of each trust's obligations in respect of its trust preferred securities. See "Relationship Among Trust Preferred Securities, Debt Securities and Guarantees."

Status of the Guarantees

        Each guarantee will constitute an unsecured obligation of ours. The applicable prospectus supplement will describe the ranking of each guarantee.

        Each guarantee will constitute a guarantee of payment and not of collection; specifically, the guaranteed party may institute a legal proceeding directly against the guarantor to enforce its rights under the guarantee without first instituting a legal proceeding against any other person or entity. Each guarantee will be held by the respective guarantee trustee for the benefit of the holders of the related trust preferred securities. A guarantee will not be discharged except by payment of the applicable guarantee payments in full to the extent not paid or distributed by the respective trust.

Amendments and Assignment

        Except with respect to any changes that do not materially adversely affect the rights of holders of the related trust preferred securities, in which case no vote will be required, a guarantee may not be amended without the prior approval of the holders of not less than a majority of the aggregate liquidation amount of the related trust preferred securities. The manner of obtaining this type of approval will be as set forth under "Description of Trust Preferred Securities—Voting Rights; Amendment of Trust Agreement." All guarantees and agreements contained in each guarantee shall bind the successors, assigns, receivers, trustees and representatives of ours and shall inure to the benefit of the holders of the related trust preferred securities then outstanding.

Events of Default

        An event of default under a guarantee will occur upon our failure to perform any of our payment or other obligations under the guarantee, or to perform any other obligation if such default remains unremedied for 30 days.

        The holders of not less than a majority in aggregate liquidation amount of the related trust preferred securities have the right to direct the time, method and place of conducting any proceeding for any remedy available to the guarantee trustee in respect of the guarantee or to direct the exercise of any trust or power conferred upon the guarantee trustee under the guarantee. Any registered holder of trust preferred securities may institute a legal proceeding directly against us to enforce its rights under the related guarantee without first instituting a legal proceeding against the related trust, the guarantee trustee or any other person or entity.

        We, as guarantor, are required to file annually with each guarantee trustee a certificate as to whether or not we are in compliance with all the conditions and covenants applicable to us under each guarantee.

Consolidation, Merger, Sale of Assets and Other Transactions

        Each guarantee provides that we will not consolidate with or merge into any other corporation or sell or otherwise dispose of our properties as or substantially as an entirety to any person, unless the successor or transferee corporation or the person which receives such properties pursuant to such sale, transfer or other disposition is a corporation organized and existing under the laws of the United States

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of America, any state thereof or the District of Columbia and such successor or transferee corporation or the person which receives such properties pursuant to such sale, transfer or other disposition expressly assumes our obligations under the guarantee.

Information Concerning the Guarantee Trustee

        Each guarantee trustee, other than during the occurrence and continuance of a default by us in performance of the related guarantee, undertakes to perform only such duties as are specifically set forth in the applicable guarantee agreement. After a default with respect to the guarantee, the guarantee trustee must exercise the same degree of care and skill as a prudent person would exercise or use in the conduct of his or her own affairs. Subject to this provision, the guarantee trustee is under no obligation to exercise any of the powers vested in it by the guarantee agreement at the request of any holder of the trust preferred securities unless it is offered reasonable indemnity against the costs, expenses and liabilities that it might thereby incur.

Termination of the Guarantees

        Each guarantee will terminate and be of no further force and effect upon full payment of the redemption price of the related trust preferred securities, upon full payment of the amounts payable with respect to the trust preferred securities upon liquidation of the respective trust and upon distribution of the related debt securities to the holders of the trust preferred securities. Each guarantee will continue to be effective or will be reinstated, as the case may be, if at any time any holder of the related trust preferred securities must restore payment of any sums paid under the related trust preferred securities or the related guarantee.

Governing Law

        Each guarantee will be governed by New York law.

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RELATIONSHIP AMONG TRUST PREFERRED SECURITIES,
DEBT SECURITIES AND GUARANTEES

Full and Unconditional Guarantee

        Payments of distributions and other amounts due on the trust preferred securities issued by a trust, to the extent the trust has funds available for the payment, are irrevocably guaranteed by us as and to the extent set forth under "Description of Guarantees." Taken together, our obligations under the related debt securities, the applicable indenture, the related trust agreement (including our agreement to pay the expenses of the trust, except payments in respect of the trust securities, contained in the trust agreement) and the related guarantee provide, in the aggregate, a full, irrevocable and unconditional guarantee of payments of distributions and other amounts due on the trust preferred securities issued by a trust. No single document standing alone or operating in conjunction with fewer than all the other documents constitutes the guarantee. It is only the combined operation of these documents that has the effect of providing a full, irrevocable and unconditional guarantee of each trust's obligations in respect of the related trust preferred securities. If and to the extent that we do not make payments on the debt securities issued to a trust, the trust will not have sufficient funds to pay distributions or other amounts due on its trust preferred securities. A guarantee does not cover payment of amounts payable with respect to the trust preferred securities issued by a trust when the trust does not have sufficient funds to pay these amounts. In this event, the remedy of a holder of the trust preferred securities is to institute a legal proceeding directly against us for enforcement of payment of our obligations under debt securities having a principal amount equal to the liquidation amount of the trust preferred securities held by the holder.

Sufficiency of Payments

        As long as payments are made when due on the debt securities issued to a trust, these payments will be sufficient to cover distributions and other payments distributable on the trust preferred securities issued by that trust, primarily because:

        Notwithstanding anything to the contrary in the applicable indenture, we have the right to set-off any payment we are otherwise required to make under that indenture against and to the extent we have previously made, or are concurrently on the date of the payment making, a payment under a guarantee.

Enforcement Rights of Holders of Trust Preferred Securities

        Under the circumstances set forth under "Description of Trust Preferred Securities—Enforcement of Certain Rights by Holders of Trust Preferred Securities," holders of trust preferred securities may bring a Direct Action against us.

        A holder of any trust preferred security may institute a legal proceeding directly against us to enforce its rights under the related guarantee without first instituting a legal proceeding against the

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related guarantee trustee, the related trust or any other person or entity. See "Description of Guarantees."

Limited Purpose of Trust

        The trust preferred securities issued by a trust represent preferred undivided beneficial interests in the assets of the trust, and the trust exists for the sole purpose of issuing its trust preferred securities and trust common securities and investing the proceeds of these trust securities in debt securities. A principal difference between the rights of a holder of a trust preferred security and a holder of a debt security is that a holder of a debt security is entitled to receive from us payments on debt securities held, while a holder of trust preferred securities is entitled to receive distributions or other amounts distributable with respect to the trust preferred securities from a trust, or from us under a guarantee, only if and to the extent the trust has funds available for the payment of the distributions.

Rights Upon Dissolution

        Upon any voluntary or involuntary dissolution of a trust, other than any dissolution involving the distribution of the related debt securities, after satisfaction of liabilities to creditors of the trust as required by applicable law, the holders of the trust preferred securities issued by the trust will be entitled to receive, out of assets held by the trust, the liquidation distribution in cash. See "Description of Trust Preferred Securities—Liquidation Distribution Upon Dissolution." Since we are the guarantor under each of the guarantees and have agreed to pay for all costs, expenses and liabilities of each trust, other than each trust's obligations to the holders of the respective trust securities, the positions of a holder of trust preferred securities and a holder of debt securities relative to other creditors and to our shareholders in the event of our liquidation or bankruptcy are expected to be substantially the same.

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DESCRIPTION OF COMMON STOCK

General

        The following descriptions of our common stock and the relevant provisions of our restated articles of incorporation and by-laws are summaries and are qualified by reference to our restated articles of incorporation and by-laws which have been previously filed with the SEC and are exhibits to this registration statement, of which this prospectus is a part, as well as the applicable Missouri General and Business Corporation Law.

        Under our restated articles of incorporation, we are authorized to issue 400 million shares of common stock, $.01 par value per share, and 100 million shares of preferred stock, $.01 par value per share. At March 31, 2002, 144,200,000 shares of common stock and no shares of preferred stock were outstanding. In March 2002, we issued equity security units pursuant to which we may issue between 7,402,320 and 8,734,020 additional shares of common stock on or before May 15, 2005 in settlement of the purchase contracts comprising a portion of the units.

Dividend Rights and Limitations

        The holders of our common stock are entitled to receive such dividends as our board of directors may from time to time declare, subject to any rights of the holders of our preferred stock, if any is issued. Our ability to pay dividends depends primarily upon the ability of our subsidiaries to pay dividends or otherwise transfer funds to us. Various financing arrangements, charter provisions and regulatory requirements may impose certain restrictions on the ability of our subsidiaries to transfer funds to us in the form of cash dividends, loans or advances.

Voting Rights

        Except as otherwise provided by law and subject to the voting rights of holders of our preferred stock, the holders of our common stock have the exclusive right to vote for the election of directors and for all other purposes. Each holder of our common stock is entitled to one vote per share on all matters submitted to a vote at a meeting of shareholders, including the election of directors, which means that the holders of more than 50% of the shares voting for the election of directors can elect 100% of the directors and the holders of the remaining shares voting for the election of directors will not be able to elect any directors. The common stock shall vote together as a single class. The holders of our common stock are not entitled to cumulate votes for the election of directors. At annual and special meetings of stockholders, a majority of the outstanding shares of common stock constitutes a quorum.

Liquidation Rights

        In the event of any liquidation, dissolution or winding up of our affairs, voluntarily or involuntarily, the holders of our common stock will be entitled to receive the remainder, if any, of our assets after the payment of all our debts and liabilities and after the payment in full of any preferential amounts to which holders of any preferred stock may be entitled.

Miscellaneous

        The outstanding shares of common stock are, and the shares of common stock sold hereunder will be, upon payment for them, fully paid and nonassessable. The holders of our common stock are not entitled to any preemptive or preferential rights to subscribe for or purchase any part of any new or additional issue of stock or securities convertible into stock. Our common stock does not contain any redemption provisions or conversion rights.

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Transfer Agent and Registrar

        Ameren Services Company acts as transfer agent and registrar for the common stock.

Shareholder Rights Plan

        On October 9, 1998, our board of directors adopted a shareholder rights plan and declared a dividend of one preferred share purchase right for each outstanding share of our common stock. The plan is designed to assure shareholders of fair and equal treatment in the event of a proposed takeover. Each right entitles the registered holder to purchase from us one one-hundredth of a share of Series A junior participating preferred stock, par value $.01 per share, at an exercise price of $180 per one one-hundredth of a share of such preferred stock, subject to adjustment. The rights will become exercisable only if a person or group acquires 15% or more of our common stock or announces a tender offer, the consummation of which would result in ownership by a person or group of 15% or more of our common stock. If a person or group acquires 15% or more of our outstanding common stock, each right will entitle its holder (other than such person or members of such group) to purchase, at the right's then-current exercise price, a number of shares of our common stock having a market value of twice such price. In addition, if we are acquired in a merger or other business combination transaction after a person or group has acquired 15% or more of our outstanding common stock, each right will entitle its holder to purchase, at the right's then-current exercise price, a number of shares of the acquiring company's common stock having a market value of twice such price. The acquiring person or group will not be entitled to exercise these rights.

        The SEC approved the plan under the Public Utility Holding Company Act of 1935 in December 1998. The rights were issued as a dividend payable January 8, 1999, to shareholders of record on that date. The rights will expire on October 9, 2008. One right will accompany each new share of our common stock issued prior to such expiration date. The rights do not have voting or dividend rights, and until they become exercisable, have no dilutive effect on our per-share earnings.

        We have 4 million shares of preferred stock initially reserved for issuance upon exercise of the rights. There is no junior participating preferred stock issued or outstanding as of the date of this prospectus.

        The description and terms of the rights are set forth in an agreement between us and EquiServe Trust Company, N.A. (successor to First Chicago Trust Company of New York), as rights agent. The proceeding summary of the rights and the shareholder rights plan is qualified in its entirety by reference to the rights agreement and the description thereof each contained in our registration statement on Form 8-A dated November 23, 1998, which is incorporated by reference into this prospectus.

Certain Anti-Takeover Matters

        Our restated articles of incorporation and by-laws include a number of provisions that may have the effect of discouraging persons from acquiring large blocks of our stock or delaying or preventing a change in our control. The material provisions that may have such an effect include:

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        In addition, the Missouri General and Business Corporation Law, or the MGBCL, contains certain provisions, including business combination provisions that would be applicable to certain mergers, share exchanges or sales of substantially all assets involving us or a subsidiary and a significant shareholder and which could have the effect of substantially increasing the cost to the acquiror and thus discouraging any such transaction. The MGBCL permits shareholders to adopt an amendment to the articles of incorporation opting out of the business combination provisions, and our restated articles of incorporation opt out of such provisions.

        Under the Illinois Public Utilities Act, approval of the Illinois Commerce Commission is required for any transaction which, regardless of the means by which it is accomplished, results in a change in the ownership of a majority of the voting capital stock of an Illinois public utility or the ownership or control of any entity which owns or controls a majority of the voting capital stock of a public utility. Because we control a majority of the voting stock of AmerenCIPS and AmerenUE, both public utilities subject to Illinois utility regulation, any change in our ownership or control, within the meaning of the Illinois Public Utilities Act, would require Illinois Commerce Commission approval. Certain acquisitions by any person of our outstanding voting shares would also require approval of the SEC under the Public Utility Holding Company Act of 1935.


DESCRIPTION OF STOCK PURCHASE CONTRACTS AND STOCK PURCHASE UNITS

        We may issue stock purchase contracts, including contracts obligating holders to purchase from us, and us to sell to the holders, a specified number of shares of our common stock at a future date or dates, or warrants. The price per share of common stock and the number of shares of common stock may be fixed at the time the stock purchase contracts or warrants are issued or may be determined by reference to a specific formula set forth in the stock purchase contracts. The stock purchase contracts may be issued separately or as part of units, often known as stock purchase units, consisting of a stock purchase contract and beneficial interests in:

securing the holders' obligations to purchase the common stock under the stock purchase contracts. The stock purchase contracts may require us to make periodic payments to the holders of the stock purchase units or vice versa, and these payments may be unsecured or prefunded on some basis. The stock purchase contracts may require holders to secure their obligations under those contracts in a specified manner.

        The applicable prospectus supplement will describe the terms of the stock purchase contracts, stock purchase units or warrants, including, if applicable, collateral or depositary arrangements.

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BOOK-ENTRY SYSTEM

        Unless otherwise indicated in the applicable prospectus supplement, each series of debt securities (other than debt securities issued to a trust) and trust preferred securities will initially be issued in the form of one or more global securities, in registered form, without coupons. The global security will be deposited with, or on behalf of, the depository, and registered in the name of the depository or a nominee of the depository. Unless otherwise indicated in the applicable prospectus supplement, the depository for any global securities will be The Depository Trust Company, or DTC.

        So long as the depository, or its nominee, is the registered owner of a global security, such depository or such nominee, as the case may be, will be considered the owner of such global security for all purposes under the applicable indenture, including for any notices and voting. Except in limited circumstances, the owners of beneficial interests in a global security will not be entitled to have securities registered in their names, will not receive or be entitled to receive physical delivery of any such securities and will not be considered the registered holder thereof under the applicable indenture. Accordingly, each person holding a beneficial interest in a global security must rely on the procedures of the depository and, if such person is not a direct participant, on procedures of the direct participant through which such person holds its interest, to exercise any of the rights of a registered owner of such security.

        Global securities may be exchanged in whole for certificated securities only if:

In any such case, we have agreed to notify the applicable trustee in writing that, upon surrender by the direct participants and indirect participants of their interest in such global securities, certificated securities representing the applicable securities will be issued to each person that such direct participants and indirect participants and the depository identify as being the beneficial owner of such securities.

        The following is based solely on information furnished by DTC:

        DTC will act as depository for the global securities. The global securities will be issued as fully-registered securities registered in the name of Cede & Co., DTC's partnership nominee. One fully-registered global security certificate will be issued for each issue of the global securities, each in the aggregate principal amount of such issue and will be deposited with DTC.

        DTC is a limited-purpose trust company organized under the New York Banking Law, a "banking organization" within the meaning of the New York Banking Law, a member of the Federal Reserve System, a "clearing corporation" within the meaning of the New York Uniform Commercial Code, and a "clearing corporation" registered pursuant to the provisions of Section 17A of the Securities Exchange Act of 1934. DTC holds securities that its direct participants deposit with DTC. DTC also facilitates the settlement among direct participants of securities transactions, such as transfers and pledges, in deposited securities through electronic computerized book-entry changes in direct participants' accounts, thereby eliminating the need for physical movement of securities certificates.

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        Direct participants include securities brokers and dealers, banks, trust companies, clearing corporations and certain other organizations. DTC is owned by a number of its direct participants and by the New York Stock Exchange, Inc., the American Stock Exchange, Inc. and the National Association of Securities Dealers, Inc. Access to the DTC system is also available to others such as securities brokers and dealers, banks and trust companies that clear through or maintain a custodial relationship with a direct participant, either directly or indirectly, which are referred to as indirect participants and, together with the direct participants, the participants. The rules applicable to DTC and its participants are on file with the SEC.

        Purchases of global securities under the DTC system must be made by or through direct participants, who will receive a credit for such purchases of global securities on DTC's records. The ownership interest of each actual purchaser of each global security, or beneficial owner, is in turn to be recorded on the direct and indirect participants' records. Beneficial owners will not receive written confirmation from DTC of their purchase, but beneficial owners are expected to receive written confirmations providing details of the transaction, as well as periodic statements of their holdings, from the direct or indirect participant through which the beneficial owner entered into the transaction. Transfers of ownership interests in the global securities are to be accomplished by entries made on the books of participants acting on behalf of beneficial owners. Beneficial owners will not receive certificates representing their ownership interests in the global securities, except in the event that use of the book-entry system for the global securities is discontinued.

        To facilitate subsequent transfers, all global securities deposited by participants with DTC are registered in the name of DTC's partnership nominee, Cede & Co. The deposit of global securities with DTC and their registration in the name of Cede & Co. effect no change in beneficial ownership. DTC has no knowledge of the actual beneficial owners of the global securities; DTC's records reflect only the identity of the direct participants to whose accounts such global securities are credited which may or may not be the beneficial owners. The participants will remain responsible for keeping account of their holdings on behalf of their customers.

        Conveyance of notices and other communications by DTC to direct participants, by direct participants to indirect participants, and by direct participants and indirect participants to beneficial owners will be governed by arrangements among them, subject to any statutory or regulatory requirements as may be in effect from time to time.

        If the global securities are redeemable, redemption notices shall be sent to Cede & Co. If less than all of the global securities are being redeemed, DTC's practice is to determine by lot the amount of the interest of each direct participant in such issue to be redeemed.

        Neither DTC nor Cede & Co. will consent or vote with respect to the global securities. Under its usual procedures, DTC mails an omnibus proxy to us as soon as possible after the record date. The omnibus proxy assigns Cede & Co.'s consenting or voting rights to those direct participants whose accounts the global securities are credited on the record date, identified in a listing attached to the omnibus proxy.

        Principal, interest and premium payments, if any, on the global securities will be made to DTC in immediately available funds. DTC's practice is to credit direct participants' accounts on the date on which interest is payable in accordance with the respective holdings shown on DTC's records, unless DTC has reason to believe that it will not receive payment on such date. Payments by participants to beneficial owners will be governed by standing instructions and customary practices, as is the case with securities held for the accounts of customers in bearer form or registered in "street name," and will be the responsibility of such participant and not of DTC, the trustee for such securities, or us, subject to any statutory or regulatory requirements as may be in effect from time to time. Payment of principal, interest and premium, if any, on any of the aforementioned securities represented by global securities to DTC is the responsibility of the appropriate trustee and us. Disbursement of such payments to direct

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participants shall be the responsibility of DTC, and disbursement of such payments to the beneficial owners shall be the responsibility of the participants.

        The information in this section concerning DTC and DTC's book-entry system has been obtained from sources, including DTC, that we believe to be reliable, but we take no responsibility for the accuracy thereof.

        The underwriters, dealers or agents of any of the securities may be direct participants of DTC.

        None of the trustees, us or any agent for payment on or registration of transfer or exchange of any global security will have any responsibility or liability for any aspect of the records relating to or payments made on account of beneficial interests in such global security or for maintaining, supervising or reviewing any records relating to such beneficial interests.

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PLAN OF DISTRIBUTION

        We may sell the securities:

        The applicable prospectus supplement will set forth the terms under which the securities are offered, including the name or names of any underwriters, the purchase price of the securities and the proceeds to us from the sale, any underwriting discounts and other items constituting underwriters' compensation, any initial offering price and any discounts, commissions or concessions allowed or reallowed or paid to dealers.

        Any initial offering price and any discounts, concessions or commissions allowed or reallowed or paid to dealers may be changed from time to time.

        If underwriters are used in an offering, the securities will be acquired by the underwriters for their own account and may be resold from time to time in one or more transactions, including negotiated transactions, at a fixed public offering price or at varying prices determined at the time of sale. The securities may be offered to the public either through underwriting syndicates represented by one or more managing underwriters or directly by one or more of those firms. The specific managing underwriter or underwriters, if any, will be named in the prospectus supplement relating to the particular securities together with the members of the underwriting syndicate, if any. Unless otherwise set forth in the applicable prospectus supplement, the obligations of the underwriters to purchase the particular securities will be subject to certain conditions precedent and the underwriters will be obligated to purchase all of the securities being offered if any are purchased.

        We may sell the securities directly or through agents we designate from time to time. The applicable prospectus supplement will set forth the name of any agent involved in the offer or sale of the securities in respect of which such prospectus supplement is delivered and any commissions payable by us to such agent. Unless otherwise indicated in the applicable prospectus supplement, any agent will be acting on a best efforts basis for the period of its appointment.

        Any underwriters utilized may engage in stabilizing transactions and syndicate covering transactions in accordance with Rule 104 of Regulation M under the Securities Exchange Act of 1934. Stabilizing transactions permit bids to purchase the underlying security so long as the stabilizing bids do not exceed a specified maximum. Syndicate covering transactions involve purchases of the particular offered securities in the open market after the distribution has been completed in order to cover syndicate short positions. These stabilizing transactions and syndicate covering transactions may cause the price of the offered securities to be higher than it would otherwise be in the absence of such transactions.

        Any underwriters, dealers or agents participating in the distribution of the securities may be deemed to be underwriters and any discounts or commissions received by them on the sale or resale of the securities may be deemed to be underwriting discounts and commissions under the Securities Act. Agents, dealers and underwriters may be entitled, under agreements entered into with us, to indemnification by us against certain liabilities, including liabilities under the Securities Act, and to contribution with respect to payments which the agents, dealers or underwriters may be required to make in respect of these liabilities. Agents, dealers and underwriters may engage in transactions with or perform services for us in the ordinary course of business.

        Unless otherwise specified in a prospectus supplement, except for our common stock, which is listed on the New York Stock Exchange, the securities will not be listed on a national securities

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exchange or the Nasdaq Stock Market. No assurance can be given that any broker-dealer will make a market in any series of the securities, and, in any event, no assurance can be given as to the liquidity of the trading market for any of the securities. The prospectus supplement will state, if known, whether or not any broker-dealer intends to make a market in the securities. If no such determination has been made, the prospectus supplement will so state.


LEGAL MATTERS

        Steven R. Sullivan, Esq., our Vice President, General Counsel and Secretary, will pass upon the validity of the offered securities (except the trust preferred securities) for us and Richards, Layton & Finger, P.A., as special Delaware counsel for Ameren Capital Trust I, Ameren Capital Trust II and us, will pass upon the validity of the trust preferred securities for each trust and us. Pillsbury Winthrop LLP, New York, New York, will pass upon the validity of the offered securities for any underwriters, dealers, purchasers or agents. Pillsbury Winthrop LLP represents us from time to time in connection with various matters.


EXPERTS

        The consolidated financial statements as of December 31, 2001 and 2000, and for each of the three years in the period ended December 31, 2001, incorporated in this prospectus by reference to our Current Report on Form 8-K dated February 14, 2002 have been so incorporated in reliance upon the report of PricewaterhouseCoopers LLP, independent accountants, given on the authority of said firm as experts in auditing and accounting.

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        No dealer, salesperson or other person is authorized to give any information to or represent anything not contained in this prospectus supplement or the accompanying prospectus. You must not rely on any unauthorized information or representations. This prospectus supplement and the accompanying prospectus is an offer to sell only the shares offered hereby, but only under circumstances and in jurisdictions where it is lawful to do so. The information contained in this prospectus supplement and the accompanying prospectus is current only as of their respective dates.

TABLE OF CONTENTS

Prospectus Supplement

 
   Page
Prospectus Supplement Summary   S-2
Risk Factors   S-8
Use of Proceeds   S-16
Capitalization   S-17
Price Range of Common Stock and Dividends   S-18
Underwriting   S-19
Legal Matters   S-20
Experts   S-20

Prospectus

Risk Factors   3
Ameren Corporation   9
Ameren Capital Trust I and Ameren Capital Trust II   10
Where You Can Find More Information   11
Ratios of Earnings to Fixed Charges   12
Use of Proceeds   12
Forward-Looking Statements   13
About This Prospectus   15
Description of Debt Securities   16
Description of Trust Preferred Securities   26
Description of Guarantees   41
Relationship Among Trust Preferred Securities, Debt Securities and Guarantees   44
Description of Common Stock   46
Description of Stock Purchase Contracts and Stock Purchase Units   48
Book-Entry System   49
Plan of Distribution   52
Legal Matters   53
Experts   53

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Ameren Corporation

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PROSPECTUS SUPPLEMENT SUMMARY
RISK FACTORS
USE OF PROCEEDS
CAPITALIZATION
PRICE RANGE OF COMMON STOCK AND DIVIDENDS
UNDERWRITING
LEGAL MATTERS
EXPERTS
TABLE OF CONTENTS
RISK FACTORS
AMEREN CORPORATION
AMEREN CAPITAL TRUST I AND AMEREN CAPITAL TRUST II
WHERE YOU CAN FIND MORE INFORMATION
RATIOS OF EARNINGS TO FIXED CHARGES
USE OF PROCEEDS
FORWARD-LOOKING STATEMENTS
ABOUT THIS PROSPECTUS
DESCRIPTION OF DEBT SECURITIES
DESCRIPTION OF TRUST PREFERRED SECURITIES
DESCRIPTION OF GUARANTEES
RELATIONSHIP AMONG TRUST PREFERRED SECURITIES, DEBT SECURITIES AND GUARANTEES
DESCRIPTION OF COMMON STOCK
DESCRIPTION OF STOCK PURCHASE CONTRACTS AND STOCK PURCHASE UNITS
BOOK-ENTRY SYSTEM
PLAN OF DISTRIBUTION
LEGAL MATTERS
EXPERTS