Exhibit 2.2



                               AMENDMENT NO. 3 TO
                            STOCK PURCHASE AGREEMENT


     THIS  AMENDMENT  NO. 3,  dated as of May 31,  2004,  to the STOCK  PURCHASE
AGREEMENT,  dated as of February 2, 2004,  is entered  into by and among  Ameren
Corporation,  a Missouri corporation  ("Purchaser"),  Illinova  Corporation,  an
Illinois  corporation  ("Seller"),  Illinova  Generating  Company,  an  Illinois
corporation  ("IGC"),  and Dynegy  Inc.,  an  Illinois  corporation  ("Dynegy").
Dynegy, IGC and Seller are referred to herein as the "Dynegy Parties".

                              W I T N E S S E T H:

     WHEREAS,  Purchaser and the Dynegy  Parties  entered into a Stock  Purchase
Agreement,  dated  February 2, 2004,  as amended by Amendment  No. 1 dated as of
March 23, 2004 and by Amendment  No. 2 dated as of April 30, 2004 (the  "Amended
Agreement"),  providing for the sale to Purchaser of all of the capital stock of
Illinois Power Company, an Illinois  corporation,  held by Seller, and IGC's 20%
share of Electric Energy, Inc., an Illinois corporation; and

     WHEREAS,  Purchaser  and the  Dynegy  Parties  wish to  amend  the  Amended
Agreement as set forth herein;

     NOW,  THEREFORE,  in  consideration  of the premises and the mutual  terms,
conditions and  agreements set forth herein,  the parties hereto hereby agree as
follows:

          Section 1 Defined Terms.  All  capitalized  terms used and not defined
     herein have the meanings set forth in the Amended Agreement.

          Section 2 Amendment to Section  5.21.  Section  5.21(b) of the Amended
     Agreement is amended by changing the  reference to "120 days" in the second
     sentence to "150 days".

          Section 3 Amendment to Exhibit B.  Exhibit B to the Amended  Agreement
     is amended by changing the reference to "one hundred  twenty (120) days" in
     the bracketed note at the top of page 1 of Exhibit B to "one hundred thirty
     five (135) days".

          Section 4 No Other Amendments. Except as set forth herein, the Amended
     Agreement remains in full force and effect.

          Section 5 Counterparts.  This Agreement may be executed in one or more
     counterparts,  and by the parties in separate  counterparts,  each of which
     when  executed  shall be deemed to be an  original  but all of which  taken
     together shall constitute one and the same agreement.



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     IN WITNESS  WHEREOF,  Seller,  IGC,  Dynegy and Purchaser  have caused this
Amendment  No. 3 to the  Original  Agreement to be executed as of the date first
written above by their respective officers thereunto duly authorized.

                                ILLINOVA CORPORATION


                                By      /s/ Alec Dreyer
                                  ----------------------------------------------
                                  Name:
                                  Title:


                                ILLINOVA GENERATING COMPANY


                                By      /s/ Alec Dreyer
                                  ----------------------------------------------
                                  Name:
                                  Title:


                                DYNEGY INC.


                                By      /s/ Alec Dreyer
                                  ----------------------------------------------
                                  Name:
                                  Title:


                                AMEREN CORPORATION


                                By      /s/ Steven R. Sullivan
                                  ----------------------------------------------
                                  Name:
                                  Title: