Exhibit 10.91

                            INSTRUMENT OF ASSUMPTION

         INSTRUMENT OF ASSUMPTION dated as of December 15, 1999, by MIDWEST
GENERATION, LLC, a Delaware limited liability company ("Purchaser"), in favor of
COMMONWEALTH EDISON COMPANY, an Illinois corporation ("ComEd"), and UNICOM
INVESTMENT INC., an Illinois corporation ("UII").

                              W I T N E SS E T H :

         WHEREAS, pursuant to the Asset Sale Agreement dated as of March 22,
1999 (as amended and/or assigned, the "Agreement") between Edison Mission
Energy, a California corporation, and ComEd and that certain Asset Sale
Agreement dated as of May 11, 1999 between ComEd and UII, ComEd and UII are
concurrently herewith selling, assigning, transferring, conveying and delivering
to Purchaser certain assets and properties defined in the Agreement as the
Assets; and

         WHEREAS, in partial consideration for such sale, assignment, transfer,
conveyance and delivery of the Assets, the Agreement requires Purchaser to
perform, pay or discharge certain obligations of ComEd, which are defined in the
Agreement as the Purchaser's Liabilities, by the execution and delivery to ComEd
and UII of this Instrument of Assumption;

         NOW, THEREFORE, pursuant to the terms of the Agreement and for good and
valuable consideration, Purchaser hereby assumes and undertakes and agrees to
perform, pay or discharge in accordance with the terms and conditions thereof,
all of the Purchaser's Liabilities (as defined in the Agreement).

         AND, for the consideration aforesaid, Purchaser for itself and its
successors and assigns has covenanted and by this instrument does covenant with
ComEd, UII and their respective successors and assigns that Purchaser and its
successors and assigns will do, execute and deliver, or will cause to be done,
executed and delivered, all such further acts, transfers, assignments and
conveyances, powers of attorney and assurances for the better assuming from
ComEd, UII and their respective successors and assigns all of the Purchaser's
Liabilities as ComEd, UII and their respective successors or assigns shall
reasonably require.

         Capitalized terms used herein without definition shall have the
respective meanings ascribed to such terms in the Agreement.




         This Instrument of Assumption shall inure to the benefit of and be
binding upon the successors and assigns of ComEd and UII and the successors and
assigns of Purchaser.


                                       MIDWEST GENERATION, LLC

                                       By:   /s/ Georgia Nelson
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