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                                                                    Exhibit 10.5

                              ASSUMPTION AGREEMENT

          ASSUMPTION AGREEMENT (this "AGREEMENT"), dated as of October 5, 2004,
is made by BCP Crystal US Holdings Corp., a Delaware corporation (the "ASSUMING
PARTY"), and delivered to Deutsche Bank AG, New York Branch, as Administrative
Agent and as Collateral Agent, for the Secured Parties (as defined in the Credit
Agreement referred to below) (in such capacities, the "AGENT"). Unless otherwise
defined herein, capitalized terms used herein shall have the meanings provided
such terms in the Credit Agreement.

                              W I T N E S S E T H :

          WHEREAS, BCP Crystal Holdings Ltd. 2, a company incorporated with
limited liability under the laws of the Cayman Islands ("HOLDINGS"), BCP Caylux
Holdings Luxembourg S.C.A., a corporate partnership limited by shares (societe
en commandite par actions) organized under the laws of Luxembourg ("PARENT"),
Celanese Americas Corporation, a Delaware corporation ("CAC"), certain
subsidiaries of Parent from time to time party thereto as a borrower under the
Revolving Facility provided for therein, the Lenders party thereto from time to
time, Morgan Stanley Senior Funding, Inc., as global coordinator, Deutsche Bank
AG, New York Branch, as administrative agent (in such capacity, the
"ADMINISTRATIVE AGENT") and as collateral agent (in such capacity, the
"COLLATERAL AGENT") for the Lenders, and Deutsche Bank AG, New York Branch and
Morgan Stanley Senior Funding, Inc., as joint lead arrangers are parties to a
Credit Agreement, dated as of April 6, 2004 (as amended, modified or
supplemented from time to time, the "CREDIT AGREEMENT");

          WHEREAS, in connection with the Credit Agreement, CAC and certain of
its Subsidiaries have entered into a Guarantee and Collateral Agreement, dated
as of April 6, 2004 (as in effect from time to time, the "US COLLATERAL
AGREEMENT" and, together with the Credit Agreement, the "DOCUMENTS");

          WHEREAS, as a result of the Restructuring , the Assuming Party will
become the Term Borrower under the Credit Agreement;

          WHEREAS, the Assuming Party desires to execute and deliver this
Agreement and to become a party to each of the Documents in order to satisfy the
requirements set forth in the definitions of "Parent Merger" and "Restructuring"
contained in Article I of the Credit Agreement;

          NOW, THEREFORE, IT IS AGREED:

          1.     CREDIT AGREEMENT. By executing and delivering this Agreement,
which Agreement shall for all purposes be deemed to constitute a counterpart to
the Credit Agreement, the Assuming Party hereby becomes the Term Borrower for
all purposes under the Credit Agreement and hereby expressly assumes all
Obligations thereunder, and will be bound by all terms, conditions and duties
applicable to the Term Borrower under the Credit Agreement and the other Loan
Documents.

          2.     US COLLATERAL AGREEMENT. Attached hereto as ANNEX I is the
supplement to the US Collateral Agreement in the form of Exhibit I to the US
Collateral Agreement executed

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and delivered by the Assuming Party to the Collateral Agent on the date hereof.

          3.     COVENANTS, REPRESENTATIONS, ETC. Without limiting the
foregoing, the Assuming Party hereby (i) represents and warrants that the
representations and warranties made by, and as the Term Borrower pursuant to
Article III of the Credit Agreement, as of the date hereof and all other Loan
Documents to which it is a party are true and correct, in all material respects,
on and as of the date hereof and (ii) undertakes each covenant made by, and as
the Term Borrower pursuant to Article V and Article VI of the Credit Agreement,
as of the date hereof and all other Loan Documents to which it is or becomes a
party.

          4.     LOAN DOCUMENT. From and after the execution and delivery hereof
by the parties, this Agreement shall constitute a "Loan Document" for all
purposes of the Credit Agreement and the other Loan Documents.

          5.     COUNTERPARTS. This Agreement may be signed in any number of
counterparts, each of which shall be an original, with the same effect as if the
signatures thereto and hereto were upon the same instrument.

          6.     RELEASE. Upon the acceptance hereof by the Agent, Parent is
hereby fully and completely released as the Term Borrower under the Credit
Agreement and shall have no further obligations thereunder as the Term Borrower.

          7.     SUBSIDIARIES. Attached hereto as ANNEX II is a Schedule of the
corporate structure of Holdings and its Subsidiaries on the Restructuring Date
as required pursuant to Section 3.08(a) of the Credit Agreement.

          8.     GOVERNING LAW. THIS AGREEMENT SHALL BE GOVERNED BY AND
CONSTRUED IN ACCORDANCE WITH THE LAW OF THE STATE OF NEW YORK.


                                     * * * *

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          IN WITNESS WHEREOF, each of the undersigned has caused this Agreement
to be duly executed and delivered as of the date first above written.


                                          BCP CRYSTAL US HOLDINGS CORP.,

                                          By /s/ Chinh E. Chu
                                             ------------------------
                                             Name: Chinh E. Chu
                                             Title: President


Agreed to and Acknowledged:

DEUTSCHE BANK AG, NEW YORK BRANCH,
as Administrative Agent and Collateral Agent

By /s/ Carin M. Keegan
   --------------------------------
   Name: Carin M. Keegan
   Title: Vice President


By /s/ Susan LeFevre
   --------------------------------
    Name: Susan LeFevre
    Title: Director