<PAGE>

                                                                    Exhibit 99.5

                                  [LOGO] HARKEN
                               Energy corportion

                             Shares of Common Stock

                           Offered Pursuant to Rights

                         Distributed to Security Holders

                 , 2003
-----------------

To Our Clients:

     Enclosed for your consideration are a prospectus, dated                 ,
                                                             ----------------
2003 (the "Prospectus"), and the "Instructions as to Use of Harken Energy
Corporation Rights Certificates" relating to the offering (the "Rights
Offering") by Harken Energy Corporation (the "Company") of shares of its Common
Stock (as defined below) pursuant to non-transferable subscription rights (the
"Rights") distributed to all holders of record of shares of its common stock,
par value $0.01 per share (the "Common Stock"), and shares of its Series G1
preferred stock and Series G2 preferred stock (collectively, the "Preferred
Stock") at the close of business on               , 2003 (the "Record Date").
                                    --------------
The Rights are described in the Company's Prospectus dated                , 2003
                                                           ---------------
(the "Prospectus").

     In the Rights Offering, the Company is offering an aggregate of
                                                                     -------
shares of Common Stock (the "Underlying Shares"), as described in the
Prospectus.

     The Rights will expire, if not exercised, at 5:00 p.m., New York City time,
on               , 2003, unless extended in the sole discretion of the Company
   --------------
(as it may be extended, the "Expiration Date").

     As described in the Prospectus, you will receive one Right for each share
of Common Stock owned (or in the case of Preferred Stock, one Right for each
share of Common Stock issuable upon conversion) of record as of the close of
business on the Record Date.

     Each Right allows you to subscribe for       shares of Common Stock. The
                                            -----
Subscription Price for the Rights is $    per Right (or $    per share of Common
                                      ---                ---
Stock) (the "Subscription Right").

     If any shares of Common Stock offered in the Rights Offering remain
unsubscribed after the Rights Offering, Lyford Investments Enterprises Ltd., a
creditor of the Company, has agreed to purchase all such shares at the
Subscription Price.

     The Rights will be evidenced by non-transferable Rights certificates and
will cease to have any value at the close of business on the Expiration Date.

     The materials enclosed are being forwarded to you as the beneficial owner
of Common Stock carried by us in your account but not registered in your name.
Exercises and sales of rights may be made only by us as the record owner and
pursuant to your instructions. Accordingly, we request instructions as to
whether you wish us to elect to subscribe for any shares of Common Stock to
which you are entitled pursuant to the terms and subject to the conditions set
forth in the enclosed

                                       1

<PAGE>

Prospectus. However, we urge you to read the document carefully before
instructing us to exercise your Rights.

     Your instructions to us should be forwarded as promptly as possible in
order to permit us to exercise Rights on your behalf in accordance with the
provisions of the Rights Offering. The Rights Offering will expire at 5:00 p.m.,
New York City time, on the Expiration Date. Once you have exercised the
Subscription Right, such exercise may not be revoked.

     If you wish to have us, on your behalf, exercise the Rights for any shares
of Common Stock to which you are entitled, please so instruct us by completing,
executing and returning to us the instruction form on the reverse side of this
letter.

     Any questions or requests for assistance concerning the Rights Offering
         should be directed to American Stock Transfer & Trust Company,
                   the Subscription Agent, at (800)    -    .
                                                    --- ----

                                       2

<PAGE>

                         Beneficial Owner Election Form

                                  Instructions

     The undersigned acknowledge(s) receipt of your letter and the enclosed
materials referred to therein relating to the offering of shares of common stock
(the "Common Stock") of Harken Energy Corporation (the "Company").

     This will instruct you whether to exercise Rights to purchase shares of the
Company's Common Stock distributed with respect to the shares of the Company's
Common Stock held by you for the account of the undersigned, pursuant to the
terms and subject to the conditions set forth in the Prospectus and the related
"Instructions as to Use of Harken Energy Corporations Rights Certificates."

     Box 1. [ ]   Please DO NOT EXERCISE RIGHTS for shares of Common Stock.

     Box 2. [ ]   Please EXERCISE RIGHTS for shares of Common Stock as set forth
                  below.

                          Number of       Subscription
                           Rights            Price           Payment
                        -------------   ----------------   -----------
Subscription Right                    x                  =

     Box 3. [ ]   Payment in the following amount is enclosed $                .
                                                               ----------------
                  (This amount must equal the amount set forth under "Payment"
                  above).

     Box 4. [ ]   Please deduct payment from the following account maintained by
                  you as follows:

----------------------------------           -----------------------------------
         Type of Account                                  Account No.
                                             $
                                              ----------------------------------

                                             -----------------------------------

                                             -----------------------------------
                                             Signature(s)

                                             Please type or print name(s) below:

                                             -----------------------------------

                                             -----------------------------------

Date:                  , 2003
     ------------------

                                       3