Exhibit 99




                                                       July 17, 2000





VIA FAX AND OVERNIGHT MAIL



The Dow Chemical Company
2030 Dow Center
Midland, MI  48674

Attention:  Chief Executive Officer

Dear Sirs:

     Reference is made to the Agreement and Plan of Merger, dated
as of August 3, 1999 (the "Agreement"), among Union Carbide
Corporation, a New York corporation, The Dow Chemical Company, a
Delaware corporation, and Transition Sub Inc., a Delaware
corporation.

     Each of the parties hereto agrees that it will not
exercise any right it may have to terminate the Agreement under
Section 8.2(i) of the Agreement prior to September 30, 2000.
Notwithstanding anything to the contrary in the foregoing, this
letter agreement shall not in any way limit, restrict or affect
any of the other rights of the parties (relating to termination
of the Agreement or otherwise) under the Agreement.


                              - 2 -


     If the foregoing is acceptable to you, please execute a copy
of this letter agreement in the space below, whereupon this
instrument will constitute a binding agreement among us.

                                    Very truly yours,

                                    UNION CARBIDE CORPORATION


                                    By   /s/ Bruce D. Fitzgerald
                                             Bruce D. Fitzgerald
                                             Vice President, General Counsel
                                               and Secretary


ACCEPTED AND AGREED as
of July 21, 2000:

THE DOW CHEMICAL COMPANY


By    /s/ J.P. Reinhard
   Name:  J.P. Reinhard
   Title: Executive Vice President and
            Chief Financial Officer