EXHIBIT 99.4

                               ELYO
235 AVENUE GEORGES CLEMENCEAU  BP 4601  92746 NANTERRE CEDEX

                                    January 19, 2000

Thomas R. Casten
c/o Trigen Energy Corporation
One Water Street
White Plains, NY  10601

Dear Mr. Casten:

We hereby agree to purchase (or cause an affiliate to purchase)
from you, and you hereby agree to sell to us, 1,012,402 shares
(the "Subject Shares") of common stock, par value $.01 per share
(the "Shares"), of Trigen Energy Corporation (the "Company") at a
purchase price of $23.50 per Share in cash.  The purchase and
sale shall be consummated on the 31st calendar day (the "Closing
Date") following the filing of the Schedule TO (which will
include a Schedule 13E-3, pursuant to Rule 13e-3 under the
Securities Exchange Act of 1934, as amended), by Elyo S.A. and
certain of its affiliates in connection with the offer by T
Acquisition Corp., a Delaware corporation and an indirect, wholly
owned subsidiary of Elyo S.A., to purchase any and all of the
outstanding Shares (the "Offer" to be made pursuant to the terms
of the Agreement and Plan of Merger, dated as of January 19, 2000
between Elyo S.A., T Acquisition Corp. and the Company).  If
Parent is legally barred from purchasing the Subject Shares by
reason of a court order or otherwise, Parent will buy the Subject
Shares on the date two business days following the date that such
legal prohibition ceases.

On the Closing Date, you shall deliver to us at the principal
offices of the Company the certificate(s) representing the
Subject Shares and we shall deliver to you at the principal
offices of the Company a check in an amount equal to the number
of Subject Shares times $23.50 (or, at your option specified in
writing at least 3 business days prior to the Closing Date, a
wire transfer to an account designated by you in such notice).

You hereby represent that you beneficially own 1,012,402 Subject
Shares, and that the Subject Shares constitute all of the Shares
beneficially owned, directly or indirectly, by you (other than
Shares ("Plan Shares") that may be acquired upon the exercise of
options or shares of restricted stock, in each case issued or
issuable under the Trigen Energy Corporation 1994 Stock Incentive
Plan, and Shares held by you under the Company's Section 401(k)
plan).  The Plan Shares are not Subject Shares and are not
subject to the terms and conditions of this Agreement.  In
consideration of our agreement to purchase the Subject Shares,
you hereby agree not to tender the Subject Shares pursuant to the
Offer.

Please indicate your agreement with the foregoing,
effective as of the date first above mentioned, by signing below.
This letter may be signed in any number of counterparts, each of
which shall be deemed an original, but all of which, when taken
together, shall constitute one instrument.

                              Sincerely,

                              ELYO S.A.

                              By:	/s/ Olivier Degos
                              Name:	Olivier Degos
                              Title:	Corporate Vice President
AGREED AND ACCEPTED

THOMAS R. CASTEN

/s/ Thomas R. Casten
Thomas R. Casten