May 3, 2010

Ladies and Gentlemen:

This letter is being filed in response to comments from the staff (the “Staff”) of the Securities and Exchange Commission (the “Commission”) transmitted by letter dated April 19, 2010, to Martin J. Lyons, Jr., Senior Vice President and Chief Financial Officer, of Ameren Corporation (“Ameren”), with respect to the above-referenced 2009 Form 10-K.

This letter recites each Staff comment and then provides the response to that comment.

Form 10-K for Fiscal Year Ended December 31, 2009

Item 1. Business, page 4

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Response:

In its future Form 10-K filings, commencing with the Form 10-K for the year ending December 31, 2010, we will revise Ameren’s business description in Item 1 as follows (the underlined text represents the change from the 2009 Form 10-K filing):

Ameren, headquartered in St. Louis, Missouri, is a public utility holding company under PUHCA 2005 administered by FERC. Ameren was formed in 1997 by the merger of UE and CIPSCO. Ameren acquired CILCORP in 2003 and IP in 2004. Ameren’s primary assets are the common stock of its subsidiaries, including UE, CIPS, Genco, CILCO and IP. Ameren’s subsidiaries are separate, independent legal entities with separate businesses, assets, and liabilities. Dividends on Ameren’s common stock and the payment of expenses by Ameren depend on distributions made to it by its subsidiaries. Below is a summary description of UE, CIPS, Genco, CILCO and IP. A more detailed description can be found in Note 1 - Summary of Significant Accounting Policies under Part II, Item 8:

Item 3. Legal Proceedings, page 23

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Response:

In its future Form 10-K filings (under Part I, Item 3) and Form 10-Q filings (under Part II, Item 1), commencing with the Form 10-Q for the quarter ended March 31, 2010, we will specifically reference the material pending legal proceedings that are described in other sections of the report. By way of example, the following is an excerpt from Item 3 of Ameren’s 2009 Form 10-K with clarifying language (the underlined text represents modified language from the 2009 Form 10-K filing):

We are involved in legal and administrative proceedings before various courts and agencies with respect to matters that arise in the ordinary course of business, some of which involve substantial amounts of money. We believe that the final disposition of these proceedings, except as otherwise disclosed in this report, will not have a material adverse effect on our results of operations, financial position, or liquidity. Material legal and administrative proceedings, which are discussed in Note 2—Rate and Regulatory Matters, and Note 15 - Commitments and Contingencies under Part II, Item 8, of this report and incorporated herein by reference, include the following:

Item 7. Management’s Discussion and Analysis of Financial Condition and Results of Operations, page 29

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Refer to Item 303 of Regulation S-K and SEC Release No. 33-8350.

Response:

We respectfully advise the Staff that when discussing changes in Ameren’s financial statements in Management’s Discussion and Analysis of Financial Condition and Results of Operations (MD&A), our approach is generally to identify the primary items producing such variations and the reasons for such variation. In addition, we include a separately captioned “Outlook” section in which we discuss key trends that we have identified that may affect Ameren’s financial condition, results of operations, or liquidity in the future.

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We acknowledge the Staff’s comment and will provide additional analysis concerning the quality and variability of Ameren’s earnings and cash flows, to the extent material, throughout the MD&A in our future filings. By way of example, the following are expanded analyses of the items you included in your comment from the 2009 10-K (the underlined text represents modified language from the 2009 Form 10-K filing):

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Page 42

Other operations and maintenance expenses decreased $119 million in 2009 compared with 2008 because of several factors. Coal-fired plant maintenance costs were reduced by $48 million because less work was undertaken as a result of cost containment initiatives in response to weak economic conditions. In addition, bad debt expenses were lower by $44 million because of elevated levels of bad debt expense in 2008 as a result of the transition to higher market-based rates at the Ameren Illinois Utilities and the impact of the Illinois bad debt rate adjustment mechanism (net of a related donation for customer assistance programs) discussed in Note 2—Rate and Regulatory Matters under Part II, Item 8 of this report. A favorable change of $37 million in unrealized net MTM adjustments between periods resulting from changes in the market value of investments used to support Ameren’s deferred compensation plans and the absence of a Callaway nuclear plant refueling and maintenance outage in 2009, as compared with costs of $30 million in 2008, also reduced operations and maintenance expenses. Additionally, asset impairment charges were lower by $7 million between years.

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Reducing the benefit of these items was an increase of $24 million in labor costs because of compensation increases and higher staffing levels and the recognition of $17 million for employee severance costs in 2009. In 2008, other operations and maintenance expenses were reduced by a MoPSC accounting order related to storm costs incurred in 2007, which resulted in UE recording a regulatory asset of $25 million; no similar item occurred in 2009.

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In connection with this response to the Staff’s comments, Ameren acknowledges:

If you have any questions concerning responses to these comments, please contact the undersigned at (314) 554-2574 or Martin J. Lyons, Jr., Senior Vice President and Chief Financial Officer, at (314) 554-2982.

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