<PAGE>

                           LYONDELL CHEMICAL COMPANY

              Letter to The Depository Trust Company Participants
                         for Tender of All Outstanding
               Unregistered 91/2% Senior Secured Notes due 2008
                                in Exchange for
                Registered 91/2% Senior Secured Notes due 2008

 The Exchange Offer will expire at 5:00 p.m., New York City time, on
           , 2003 (the "Expiration Date"), unless sooner terminated or
 extended. Outstanding Notes tendered in the Exchange Offer may be withdrawn at
 any time prior to 5:00 p.m., New York City time, on the Expiration Date for
 the Exchange Offer.

To Depository Trust Company Participants:

   We enclose the materials listed below relating to the offer by Lyondell
Chemical Company (the "Issuer") to exchange its 91/2% 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 its issued and outstanding unregistered 91/2% Senior Secured Notes
due 2008 (the "Outstanding Notes"), upon the terms and subject to the
conditions set forth in the Issuer's prospectus dated           , 2003 (the
"Prospectus") and the related Letter of Transmittal. The exchange of the
Outstanding Notes for the New Notes and the related documentation are referred
to herein as the "Exchange Offer."

   We are enclosing copies of the following documents:

    1. Prospectus dated           , 2003;

    2. Letter of Transmittal (together with accompanying Substitute Form W-9
       Guidelines);

    3. Notice of Guaranteed Delivery; and

    4. Letter of instructions that may be sent to your clients for whose
       account you hold Outstanding Notes in your name or in the name of your
       nominee, with space provided for obtaining such client's instructions
       with regard to the Exchange Offer.

   We urge you to contact your clients promptly. Please note that the Exchange
Offer will expire at 5:00 p.m., New York City time, on           , 2003, unless
sooner terminated or extended.

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

   Pursuant to 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 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; and

<PAGE>

  .   any New Notes received are being acquired in the ordinary course of
      business of the person receiving such New Notes.

   If such person is a broker-dealer that will receive New Notes for its own
account in exchange for Outstanding Notes, it represents 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 deliver a Prospectus in
connection with any resale of such 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.

   The enclosed Letter to Clients contains an authorization by the beneficial
owners of the Outstanding Notes for you to make the foregoing representations.

   The Issuer will not pay any fee or commission to any broker or dealer or to
any other person (other than the Exchange Agent) in connection with the
solicitation of tenders of Outstanding Notes pursuant to the Exchange Offer.
The Issuer will pay or cause to be paid any transfer taxes payable on the
transfer of Outstanding Notes to it, except as otherwise provided in
Instruction 7 of the enclosed Letter of Transmittal.

   Additional copies of the enclosed material may be obtained from us upon
request.

                                          Very truly yours,

                                          Lyondell Chemical Company

                                      2