AMEREN CORPORATION                                      One Ameren Plaza
                                                        1901 Chouteau Avenue
STEVEN R. SULLIVAN                                      PO Box 66149, MC 1300
Vice President Regulatory Policy,                       St. Louis, MO 63166-6149
General Counsel & Secretary                             314.554.2098
                                                        314.554.4014 fax


                                                                       EXHIBIT 5

November 26, 2002

Ameren Corporation
1901 Chouteau Avenue
St. Louis, Missouri 63103

Ladies and Gentlemen:

I am Vice President Regulatory Policy, General Counsel and Secretary of Ameren
Corporation, a Missouri corporation (the "Company"). The Company has filed with
the Securities and Exchange Commission (the "Commission) a Registration
Statement on Form S-8 (the "Registration Statement") under the Securities Act of
1933, as amended (the "Securities Act"), for the registration of 400,000 shares
of the Company's common stock, $.01 par value (the "Common Stock"), and the
preferred share purchase rights (the "Rights") attached thereto (the Common
Stock and the Rights herein collectively referred to as the "Shares"), in
connection with the Ameren Corporation Employee Long-Term Savings Plan - IUOE
Local No. 148 and the Ameren Corporation Employee Long-Term Savings Plan - IBEW
Local No. 702

In so acting, I have reviewed originals (or copies certified or otherwise
identified to my satisfaction) of the Registration Statement (including the
exhibits thereto), the Restated Articles of Incorporation and By-laws of the
Company as in effect on the date hereof, corporate and other documents, records
and papers and certificates of public officials. I am a member of the Bar of the
State of Missouri and, for purposes of this opinion, do not hold myself out as
an expert of the laws of any jurisdiction other than the State of Missouri.

On the basis of such review and assuming that (i) the applicable provisions of
the Securities Act and the securities or "blue sky" laws of various states shall
have been complied with, (ii) appropriate resolutions have been adopted by the
Board of Directors of the Company (or a duly appointed committee or
representative thereof), (iii) the Shares have been issued and delivered upon
the terms specified in such resolutions and in any required orders of the
Commission or other applicable regulatory approvals and (iv) the Rights shall
have been issued in accordance with the terms of the Rights Agreement, dated as
of October 9, 1998 ("Rights Agreement"), between the Company and the EquiServe
Trust Company, N.A., as Rights Agent, I am of the opinion that when the Common
Stock has been issued and delivered on the terms and conditions set forth in the
Registration Statement, and for the consideration contemplated by the
Registration Statement, the Common Stock will be duly authorized, validly
issued, fully paid and nonassessable and the Rights will have been validly
issued.

The opinion set forth above with respect to the Rights is limited to the valid
issuance of the Rights under the corporation laws of the State of Missouri. In
this connection, I have not been asked to express, and accordingly do not
express, any opinion herein with respect to any other aspect of the Rights, the
effect of any equitable principles or fiduciary considerations relating to the
adoption of the Rights Agreement or the issuance of the Rights or the
enforceability of any particular provision of the Rights Agreement.

I hereby consent to the filing of this opinion as an exhibit to the Registration
Statement. In giving this consent, I do not thereby admit that I am within the
category of persons whose consent is required under Section 7 of the Securities
Act or the rules and regulations of the Commission thereunder.

Very truly yours,

/s/ Steven R. Sullivan

Steven R. Sullivan
Vice President Regulatory Policy, General Counsel and Secretary