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                           LYONDELL CHEMICAL COMPANY

                               Letter to Clients
                                      for
                           Tender of All Outstanding

                      9 1/2% Senior Secured Notes due 2008
                           in exchange for registered
                      9 1/2% Senior Secured Notes due 2008

 The Exchange Offer will expire at 5:00 p.m., New York City time, on      ,
 2002 (the "Expiration Date"), unless sooner terminated or extended.
 Outstanding Notes tendered in the Exchange Offer may be withdrawn at any
 time before 5:00 p.m., New York City time, on the Expiration Date for the
 Exchange Offer.


To Our Clients:

   We are enclosing with this letter a prospectus dated      , 2002 (the
"Prospectus") of Lyondell Chemical Company (the "Issuer") and the related
Letter of Transmittal. These two documents together constitute the Issuer's
offer to exchange its 9 1/2% Senior Secured Notes due 2008 (the "New Notes"),
the issuance of which has been registered under the Securities Act of 1933, as
amended (the "Securities Act"), for a like principal amount of issued and
outstanding unregistered 9 1/2% Senior Secured Notes due 2008, (the
"Outstanding Notes"). The exchange of Outstanding Notes for New Notes and the
related documentation are referred to herein as the "Exchange Offer."

   The Exchange Offer is not conditioned upon any minimum aggregate principal
amount of Outstanding Notes being tendered for exchange.

   We are the holder of record of Outstanding Notes held by us for your own
account. A tender of your Outstanding Notes held by us can be made only by us
as the record holder according to your instructions. The Letter of Transmittal
is furnished to you for your information only and cannot be used by you to
tender Outstanding Notes held by us for your account.

   We request instructions as to whether you wish to tender any or all of the
Outstanding Notes held by us for your account under the terms and conditions of
the Exchange Offer. We also request that you confirm that we may, on your
behalf, make the representations contained in the Letter of Transmittal.

   Under the Letter of Transmittal, each holder of Outstanding Notes will
represent to the Issuer that:

  .   such person is not an "affiliate," as defined in Rule 405 under the
      Securities Act, of the Issuer or a broker-dealer tendering Outstanding
      Notes acquired directly from the Issuer for its own account;

  .   if such person is not a broker-dealer or is a broker-dealer but will
      not receive New Notes for its own account in exchange for Outstanding
      Notes, it is not engaged in, and does not intend to participate in, a
      distribution of the New Notes;

  .   such person does not have an arrangement or understanding with any
      person to participate in the distribution of the Outstanding Notes or
      the New Notes within the meaning of the Securities Act;

  .   any New Notes received are being acquired in the ordinary course of
      business of the person receiving such New Notes; and
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  .   if such person is a broker-dealer who will receive New Notes for its
      own account in exchange for Outstanding Notes, it will represent that
      the Outstanding Notes to be exchanged or New Notes were acquired as a
      result of market-making activities or other trading activities, and
      that it will deliver a Prospectus in connection with any resale of
      those New Notes; however, by so acknowledging and by delivering a
      Prospectus, it will not be deemed to admit that it is an "underwriter"
      within the meaning of the Securities Act.

                                          Very truly yours,

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<PAGE>

   Please return your instructions to us in the enclosed envelope within ample
time to permit us to submit a tender on your behalf before the Expiration Date.

                                 INSTRUCTION TO
                            DTC TRANSFER PARTICIPANT

To Participant of The Depository Trust Company:

   The undersigned hereby acknowledges receipt and review of the prospectus
dated      , 2002 (the "Prospectus") of Lyondell Chemical Company (the
"Issuer") and the related Letter of Transmittal. These two documents together
constitute the Issuer's offer to exchange its 9 1/2% Senior Secured Notes due
2008 (the "New Notes"), the issuance of which has been registered under the
Securities Act of 1993, as amended (the "Securities Act"), for a like principal
amount of its issued and outstanding unregistered 9 1/2% Senior Secured Notes
due 2008 (the "Outstanding Notes"). The exchange of Outstanding Notes for New
Notes and the relevant documentation are referred to herein as an "Exchange
Offer."

   This will instruct you, the registered holder and DTC participant, as to the
action to be taken by you relating to the Exchange Offer for the Outstanding
Notes held by you for the account of the undersigned.

   The aggregate principal amount of the Outstanding Notes held by you for the
account of the undersigned is (fill in amount):


<TABLE>
<CAPTION>
                                                                      Principal
   Title of Series                                                     Amount
   ---------------                                                    ---------
   <S>                                                                <C>
   Lyondell Chemical Company 9 1/2% Senior Secured Notes due 2008....
</TABLE>


   With respect to the Exchange Offer, the undersigned hereby instructs you
(check appropriate box):

[_]To TENDER all Outstanding Notes held by you for the account of the
   undersigned.

[_]To TENDER the following amount of Outstanding Notes held by you for the
   account of the undersigned:


<TABLE>
<CAPTION>
                                                                      Principal
   Title of Series                                                     Amount
   ---------------                                                    ---------
   <S>                                                                <C>
   Lyondell Chemical Company 9 1/2% Senior Secured Notes due 2008....
</TABLE>


[_]NOT to TENDER any Outstanding Notes held by you for the account of the
   undersigned.

   If no box is checked, a signed and returned Instruction to DTC Participant
will be deemed to instruct you to tender all Outstanding Notes held by you for
the account of the undersigned.

   If the undersigned instructs you to tender the Outstanding Notes held by you
for the account of the undersigned, it is understood that you are authorized to
make, on behalf of the undersigned (and the undersigned, by its signature
below, hereby makes to you), the representations contained in the Letter of
Transmittal that are to be made with respect to the undersigned as a beneficial
owner, including, but not limited to, the representations that:

  (i) the undersigned is not an "affiliate," as defined in Rule 405 under the
      Securities Act, of the Issuer or a broker-dealer tendering Outstanding
      Notes acquired directly from the Issuer for its own account;

  (ii) if the undersigned is not a broker-dealer or is a broker-dealer but
       will not receive New Notes for its own account in exchange for
       Outstanding Notes, it is not engaged in, and does not intend to
       participate in, a distribution of New Notes;

  (iii) the undersigned does not have an arrangement or understanding with
        any person to participate in the distribution of the Outstanding
        Notes or the New Notes within the meaning of the Securities Act;

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<PAGE>

  (iv) any New Notes received are being acquired in the ordinary course of
       business of the undersigned; and

  (v) if the undersigned is a broker-dealer that will receive New Notes for
      its own account in exchange for Outstanding Notes, it will represent
      that the Outstanding Notes to be exchanged for New Notes were acquired
      as a result of market-making activities or other trading activities,
      and it will acknowledge that it will deliver a Prospectus in connection
      with any resale of those New Notes; however, by so acknowledging and by
      delivering a Prospectus, the undersigned will not be deemed to admit
      that it is an "underwriter" within the meaning of the Securities Act.

                                   SIGN HERE

Name of beneficial owner(s):____________________________________________________

Signature(s):___________________________________________________________________

Name(s) (please print):_________________________________________________________

Address:________________________________________________________________________

Telephone Number:_______________________________________________________________

Taxpayer Identification or Social Security Number:______________________________

Date:___________________________________________________________________________

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