EXHIBIT F-5

                             [O'Melveny & Myers LLP]


                               September 24, 2004



Securities and Exchange Commission
450 Fifth Street, N.W.
Washington, D.C. 20549

     Re:  Ameren Corporation, et al.
          Form U-1 Application-Declaration
          (File No. 70-10220)
          --------------------------------

Ladies and Gentlemen:

     We refer to the Form U-1 Application/Declaration, as amended, in the
above-referenced proceeding (the "Application"), under the Public Utility
Holding Company Act of 1935, as amended (the "Act"), filed with the Securities
and Exchange Commission (the "Commission") by Ameren Corporation ("Ameren"), a
Missouri corporation, its indirect wholly-owned non-utility subsidiary, Ameren
Energy Fuels and Services Company ("Ameren Fuels"), an Illinois corporation, and
Illinois Power Company ("Illinois Power"), an Illinois corporation (the
"Applicants"). Capitalized terms used in this letter without definition have the
meanings ascribed to such terms in the Application.

     In the Application, Ameren is seeking authorization under the Act to
acquire the Common Shares and Preferred Shares of Illinois Power from Illinova
Corporation ("Illinova") (the "Transaction").

     We have acted as special New York counsel to Illinova and Dynegy Inc.
("Dynegy") in connection with the Transaction. In connection with this opinion,
we have examined the Application and the exhibits thereto, the Amended SPA, and
originals, or copies certified to our satisfaction, of such corporate records of
Illinois Power, Illinova and Dynegy, certificates of public officials, orders of
regulatory bodies having jurisdiction over aspects of the Transaction,
certificates of officers and representatives of Illinois Power, Illinova and
Dynegy and such other documents, records and matters of law as we have deemed
necessary for the purposes of this opinion.

     On the basis of such examination, our reliance upon the assumptions in this
opinion and our consideration of those questions of law that we have, in the
exercise of customary professional diligence, recognized as applicable to
transactions of this type, and subject to the limitations and qualifications in
this opinion, we are of the opinion that, in the event that the proposed
Transaction is consummated in accordance with the Amended SPA, all New York





State laws applicable to the proposed Transaction will have been complied with
by Illinova and Dynegy.

     The opinion expressed above is subject to the following further assumptions
and conditions:

          a. The authorizations and approvals of the Transaction by the boards
     of directors of Ameren, Illinova and Dynegy shall have been adopted and
     remain in full force and effect;

          b. All required approvals, authorizations, consents, certificates, and
     orders of, and all filings and registrations with, all applicable federal
     and state commissions and regulatory authorities with respect to the
     Transaction shall have been obtained or made, as the case may be, and shall
     remain in effect (including the approval and authorization of the
     Commission under the Act, the Federal Energy Regulatory Commission under
     the Federal Power Act, as amended, and the rules and regulations
     thereunder, and the Illinois Commerce Commission under the applicable laws
     of the State of Illinois), and the Transaction shall have been accomplished
     in accordance with all such approvals, authorizations, consents,
     certificates, orders, filings and registrations;

          c. The Commission shall have duly entered an appropriate order or
     orders with respect to the Transaction as described in the Application
     granting and permitting the Application to become effective under the Act
     and the rules and regulations thereunder;

          d. The applicable waiting period under the Hart-Scott-Rodino Antitrust
     Improvements Act of 1976, as amended, and the rules and regulations
     thereunder shall have expired with respect to the Transaction;

          e. Illinois Power, Illinova and Dynegy shall have obtained all
     authorizations, approvals, consents, waivers and releases, if any, required
     for the Transaction under all applicable laws and governing corporate
     documents, contracts, agreements, debt instruments, indentures, franchises,
     licenses and permits;

          f. No act or event other than as described herein shall have occurred
     subsequent to the date hereof which would change the opinions expressed
     herein;

          g. The Transaction shall have been consummated as described in the
     Application and all legal matters incident thereto shall be satisfactory to
     us;

          h. Each person who signed the agreements or other documents executed
     or delivered by Illinova or Dynegy in connection with the Transactions was,
     at the time of such signing, duly authorized, qualified, and acting on
     behalf of Illinova or Dynegy, as applicable, and each such person's
     signatures appearing on such agreements or other documents are genuine, and
     each such person's titles are correctly stated;

          i. The law covered by this opinion is limited to the federal law of
     the United States and the law of the State of New York, each as in effect
     on the date hereof, and we express no opinion as to the laws of any other
     jurisdiction and no opinion regarding the statutes, administrative


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     decisions, rules, regulations or requirements of any county, municipality,
     subdivision or local authority of any jurisdiction;

          j. As to various questions of fact material to such opinions we have,
     when relevant facts were not independently established, relied upon
     certificates by officers of Illinova and Dynegy and other appropriate
     persons and statements contained in the Application; and

          k. We express no opinion concerning (i) federal or state securities
     laws or regulations, (ii) federal or state antitrust, unfair competition or
     trade practice laws or regulations, (iii) pension and employee benefit laws
     and regulations, (iv) compliance with fiduciary requirements, (v) federal
     or state environmental laws and regulations, (vi) federal or state land use
     or subdivision laws or regulations or (vii) federal or state laws and
     regulations concerning filing requirements.

     We hereby consent to the use of this opinion in connection with the
Application. This opinion is intended solely for the use of the Commission and
it may not be used or relied upon by the Commission for any other purpose or by
any other person or entity, nor may copies be delivered to any other person or
entity, without in each instance our prior written consent. This opinion letter
is expressly limited to the matters set forth above, and we render no opinion,
whether by implication or otherwise, as to any other matters. We assume no
obligation to update or supplement this opinion letter to reflect any facts or
circumstances that arise after the date of this opinion letter and come to our
attention, or any future changes in laws.



                                             Very truly yours,

                                             /s/ O'Melveny & Myers LLP