<PAGE>

                                                                    Exhibit 99.7

                            Important Tax Information

          This tax information is provided in connection with the Harken Energy
Corporation (the "Company") Prospectus dated               , 2003 (the
                                             --------------
"Prospectus").

          Under the United States federal income tax laws, dividend payments
that may be made by the Company on shares of its Common Stock, par value $0.01,
issued upon the exercise of non-transferable subscription rights (the "Rights")
may be subject to backup withholding. Generally such payments will be subject to
backup withholding unless the holder (i) is exempt from backup withholding or
(ii) furnishes the payer with its correct taxpayer identification number (TIN)
and certifies that the number provided is correct and further certifies that
such holder is not subject to backup withholding as a result of a failure to
report all interest or dividend income. Each Rights holder that exercises Rights
and wants to avoid backup withholding should provide the Subscription Agent, as
the Company's agent in respect of exercised Rights (the "Requester"), with such
holder's correct taxpayer identification number (or with a certification that
such holder is awaiting a taxpayer identification number) and with a
certification that such holder is not subject to backup withholding by
completing Substitute Form W-9 below.

          Certain holders (including, among others, all corporations and certain
foreign individuals) are exempt from these backup withholding and reporting
requirements. In general, in order for a foreign individual to qualify as an
exempt recipient, that holder must submit a statement, signed under the
penalties of perjury, attesting to that individual's exempt status. Such
statements may be obtained from the Subscription Agent. Exempt holders, while
not required to file Substitute Form W-9, should file Substitute Form W-9 to
avoid possible erroneous backup withholding. See the enclosed Guidelines for
Request for Taxpayer Identification Number and Certification on Substitute Form
W-9 for additional instructions.

          If backup withholding applies, the Company or the Subscription Agent,
as the case may be, will be required to withhold up to 30% of any such dividend
payments made to a Rights holder that exercises Rights. Backup withholding is
not an additional tax. Rather, the amount of backup withholding is treated, like
any other withheld amounts, as an advance payment of the person's tax liability,
and the tax liability of persons subject to backup withholding will be reduced
by the amount of tax withheld. If withholding results in an overpayment of
taxes, a refund may be obtained.

Purpose of Substitute Form W-9

          To prevent backup withholding on dividend payments, a Rights holder
that exercises Rights is required to notify the Subscription Agent of such
holder's correct taxpayer identification number by completing Substitute Form
W-9 below and certifying on Substitute Form W-9 that the taxpayer identification
number provided is correct (or that such Rights holder is awaiting a taxpayer
identification number). In addition, the holder is required to certify on
Substitute Form W-9 that it is (i) exempt from backup withholding, or (ii) not
subject to backup withholding due to prior under reporting of interest or
dividend income, or (iii) the Internal Revenue Service (the "IRS") has notified
it that it is no longer subject to backup withholding. Further, the holder must
also certify that it is a U.S. person.

What Number to Give the Subscription Agent

          The Rights holder that exercises Rights is required to give the
Subscription Agent the taxpayer identification number of the record owner of the
shares of Common Stock issued upon the exercise of the Rights. If such record
owner is an individual, the taxpayer identification number is the taxpayer's
social security number. For most other entities, the taxpayer identification
number is the employer identification number. If the shares of Common Stock
issued upon the exercise of the Rights are in more than one name or are not in
the name of the actual owner, consult the enclosed Guidelines for Request for
Taxpayer Identification Number and Certification on Substitute Form W-9 for
additional guidelines on what number to report. If the Subscription Agent is not
provided with the correct taxpayer identification number in connection with such
payments, the holder may be subject to a penalty imposed by the IRS.

<PAGE>


<TABLE>
-----------------------------------------------------------------------------------------------------
<S>                          <C>
Requestor's Name:
Give form to the Requestor.  DO NOT send to the IRS.
-----------------------------------------------------------------------------------------------------
Name
-----------------------------------------------------------------------------------------------------
Business name, if different from above
-----------------------------------------------------------------------------------------------------
Check appropriate box:  Individual/Sole Proprietor [ ]  Corporation [ ]  Partnership [ ]  Other [ ]
-----------------------------------------------------------------------------------------------------
Address (number, street, and apt. or suite no.)
-----------------------------------------------------------------------------------------------------
City, State and ZIP code
-----------------------------------------------------------------------------------------------------
                   Part I - Taxpayer              Social Security Number
                                                                         ----------------------------
                   Identification Number (TIN)    Employer Identification Number
                   Enter your TIN in the
                   appropriate box. For most
                   individuals, this is your
                   social security number
                   (SSN). For most other
                   entities, it is your
SUBSTITUTE         employer identification
Form W-9           number (EIN).
Department of      ----------------------------------------------------------------------------------
the                Part II - If you do not have a TIN check the "TIN Applied for" box below. See "How
Internal Revenue   to Get a TIN" in the enclosed guidelines. If you are exempt from backup
Service            withholding, check the "Exempt" box below. See "Exemption from Backup Withholding"
                   in the enclosed guidelines.

                   TIN Applied for  [ ]           Exempt  [ ]
                   ----------------------------------------------------------------------------------
                   Part III - Certification
                   Under penalties of perjury, I certify that:
                   (1) The number shown on this form is my correct taxpayer identification number (or
                   I am waiting for a number to be issued to me), and
                   (2) I am not subject to backup withholding because: (a) I am exempt from backup
                   withholding; or (b) I have not been notified by the Internal Revenue Service (IRS)
                   that I am subject to backup withholding as a result of a failure to report all
                   interest or dividends; or (c) the IRS has notified me that I am no longer subject
                   to backup withholding.
                   (3) I am a U.S. person (including a U.S. resident alien).

                   Certification Instructions - You must cross out item (2) above if you have been
                   notified by the IRS that you are subject to backup withholding because you have
                   failed to report all interest and dividends on your tax return. However, if after
                   being notified by the IRS that you were subject to backup withholding you received
                   another notification from the IRS that you are no longer subject to backup
                   withholding, do not cross out item (2). The Internal Revenue Service does not
                   require your consent to any provision of this document other than the
                   certifications required to avoid backup withholding.

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


                   Signature                                          Date
                             ------------------------------                --------------------------
-----------------------------------------------------------------------------------------------------
</TABLE>

     NOTE: Failure to complete this form may result in backup withholding of up
     to 31% on any payments made to you. Please review enclosed guidelines for
     request for taxpayer identification number and certification on Substitute
     Form W-9 for additional details.

                                       2

<PAGE>

Certificate of Awaiting Taxpayer Identification Number

I certify under penalties of perjury that a taxpayer identification number has
not been issued to me, and either (a) I have mailed or delivered an application
to receive a taxpayer identification number to the appropriate Internal Revenue
Service Center or Social Security Administration Office or (b) I intend to mail
or deliver an application in the near future. I understand that if I do not
provide a taxpayer identification number to the requester a portion (not to
exceed 30%) of all reportable payments made to me will be retained and may be
remitted to the Internal Revenue Service as backup withholding until I provide a
taxpayer identification number to the requester.


Signature                                        Date
          ------------------------------              --------------------------

--------------------------------------------------------------------------------
                               Name (Please Print)

               GUIDELINES FOR REQUEST FOR TAXPAYER IDENTIFICATION
                 NUMBER AND CERTIFICATION ON SUBSTITUTE FORM W-9

What Name and Number to Give the Requester

          Name. If you are an individual, you must generally enter the name
shown on your Social Security card. However, if you have changed your last name,
for instance, due to marriage, without informing the Social Security
Administration of the name change, enter your first name, the last name shown on
your Social Security card, and your new last name. If the account is in joint
names, list first and then circle the name of the person or entity whose number
you enter in Part I of the form.

          Sole Proprietor - You must enter your individual name as shown on your
Social Security card. You may enter your business, trade or "doing business as"
name on the business name line.

          Other Entities - Enter the business name as shown on required federal
income tax documents. This name should match the name shown on the charter or
other legal document creating the entity. You may enter any business, trade or
"doing business as" name on the business name line.

          Taxpayer Identification Number (TIN). You must enter your taxpayer
identification number in the appropriate box. If you are a resident alien and
you do not have and are not eligible to get a Social Security number, your
taxpayer identification number is your IRS individual taxpayer identification
number (ITIN). Enter it in the Social Security number box. If you do not have an
individual taxpayer identification number, see "How to Get a TIN" below. If you
are a sole proprietor and you have an employer identification number, you may
enter either your Social Security number or employer identification number.
However, using your employer identification number may result in unnecessary
notices to the Requester.

          Social Security numbers (SSNs) have nine digits separated by two
hyphens: i.e. 000-00-0000. Employer identification numbers (EINs) have nine
digits separated by only one hyphen: i.e. 00-0000000.

                                       3

<PAGE>

<TABLE>
<CAPTION>
  For this type of account:        Give Name and SSN of:       For this type of account:    Give Name and SSN of:
-----------------------------   ---------------------------   ---------------------------   ----------------------
<S>                             <C>                           <C>                           <C>
1. Individual                   The individual                6. A valid trust, estate or   The legal entity (4)
                                                              pension trust
2. Two or more                  The actual owner of the       7. Corporation                The corporation
individuals (joint account)     account or, if combined
                                funds, the first individual
                                on the  account (1)
3. Custodian account of a       The minor (2)                 8. Association, club,         The organization
minor (Uniform Gift to Minors                                 religious, charitable,
Act)                                                          educational or other tax-
                                                              exempt organization
4. a. The usual revocable       The grantor-trustee (1)       9. Partnership                The partnership
savings trust (grantor is
also trustee)
b. The so-called trust          The actual owner (1)          10. A broker or registered    The broker or nominee
account that is not a legal                                   nominee
or valid trust under state
law
5. Sole proprietorship          The owner (3)                 11. Account with the          The public entity
                                                              Department of Agriculture
                                                              in the name of a public
                                                              entity (such as a state or
                                                              local government, school
                                                              district, or prison that
                                                              receives agricultural
                                                              payments)
</TABLE>

----------
(1)  List first and circle the name of the person whose number you furnish. If
     only one person on a joint account has a Social Security number, that
     person's number must be furnished.
(2)  Circle the minor's name and furnish the minor's Social Security number.
(3)  You must show your individual name, but you may also enter your business or
     "doing business as" name. You may use either your Social Security number or
     employer identification number (if you have one).
(4)  List first and circle the name of the legal trust, estate or pension trust.
     (Do not furnish the taxpayer identification number of the personal
     representative or trustee unless the legal entity itself is not designated
     in the account title.)
     Note: If no name is circled when more than one name is listed, the number
     will be considered to be that of the first name listed.

          How to Get a TIN. If you do not have a taxpayer identification number,
apply for one immediately. To apply for a Social Security number, get Form SS-5,
Application for a Social Security Number Card, from your local Social Security
Administration office. Get Form W-7 to apply for an individual taxpayer
identification number or Form SS-4, Application for Employer Identification
Number, to apply for an employer identification number. You can get Forms W-7
and SS-4 from the IRS.

          If you do not have a taxpayer identification number, check the "TIN
Applied For" box in Part II of the form and sign the Certificate of Awaiting
Taxpayer Identification Number, sign and date the form, and give it to the
Requester. For interest and dividend payments and certain payments made with
respect to readily tradable instruments, you will generally have 60 days to get
a taxpayer identification number and give it to the Requester. Other payments
are subject to backup withholding.

          Note: Checking the "TIN Applied For" box means that you have already
applied for a taxpayer identification number or that you intend to apply for one
soon.

                                       4

<PAGE>

Exemption From Backup Withholding

          Payees Exempt From Backup Withholding. Individuals (including sole
proprietors) are NOT exempt from backup withholding. Corporations are exempt
from backup withholding for certain payments, such as interest and dividends.

          For interest and dividends, the following payees are exempt from
backup withholding:

     .    A corporation;

     .    A financial institution;

     .    An organization exempt from tax under section 501(a) of the Internal
          Revenue Code of 1986, as amended (the "Code"), an individual
          retirement account (IRA), or a custodial account under section
          403(b)(7) of the Code if the account satisfies the requirements of
          section 401(f)(2) of the Code;

     .    The United States or any of its agencies or instrumentalities;

     .    A state, the District of Columbia, a possession of the United States,
          or any of their political subdivisions or instrumentalities;

     .    A foreign government or any of its political subdivisions, agencies or
          instrumentalities;

     .    An international organization or any of its agencies or
          instrumentalities;

     .    A real estate investment trust;

     .    A common trust fund operated by a bank under section 584(a) of the
          Code;

     .    An entity registered at all times during the tax year under the
          Investment Company Act of 1940;

     .    A foreign central bank of issue;

     .    A middleman known in the investment community as a nominee or who is
          listed in the most recent publication of the American Society of
          Corporate Secretaries, Inc., Nominee List; and

     .    A trust exempt from tax under section 664 of the Code or described in
          section 4947 of the Code.

          Payments Exempt From Backup Withholding. Dividends and patronage
dividends that generally are exempt from backup withholding include:

     .    Payments to nonresident aliens subject to withholding under section
          1441 of the Code;

     .    Payments to partnerships not engaged in a trade or business in the
          United States and that have at least one nonresident alien partner;

     .    Payments of patronage dividends not paid in money;

                                       5

<PAGE>

     .    Payments made by certain foreign organizations; and

     .    Payments made by an ESOP pursuant to section 404(k) of the Code.

          Interest payments that are generally exempt from backup withholding
include:

Payments of interest on obligations issued by individuals. Note, however, that
such a payment may be subject to backup withholding if this interest is $600 or
more and is paid in the course of the payer's trade or business, and you have
not provided your correct taxpayer identification number or you have provided an
incorrect taxpayer identification number to the payer;

     .    Payments of tax-exempt interest (including exempt-interest dividends
          under section 852 of the Code);

     .    Payments described in section 6049(b)(5) of the Code to nonresident
          aliens.

     .    Payments on tax-free covenant bonds under section 1451 of the Code;
          and

     .    Payments made by certain foreign organizations.

          Payments that are not subject to information reporting are also not
subject to backup withholding. For details, see sections 6041, 6041A, 6042,
6044, 6045, 6049, 6050A and 6050N of the Code, and their regulations.

          If you are exempt from backup withholding, you should still complete
and file Substitute Form W-9 to avoid possible erroneous backup withholding.
Enter your correct taxpayer identification number in Part I, check the "Exempt"
box in Part II, and sign and date the form and return it to the Requester.

          If you are a nonresident alien or a foreign entity not subject to
backup withholding, you must give the Requester a properly completed Form
W-8BEN, Certificate of Foreign Status of Beneficial Owner for United States Tax
Withholding, W-8ECI, Certificate of Foreign Person's Claim of Exemption from
Withholding on Income Effectively Connected with Conduct of a Trade or Business
in the United States, W-8EXP, Certificate of Foreign Government or Other Foreign
Organization for United States Tax Withholding or W-8IMY, Certificate of Foreign
Intermediary, Foreign Flow-Through Entity, or Certain U.S. Branches for United
States Tax Withholding. These forms can be obtained from the Requester.

Privacy Act Notice

          Section 6109 of the Code requires you to give your correct taxpayer
identification number to persons who must file information returns with the IRS
to report interest, dividends and certain other income paid to you. The IRS uses
the numbers for identification purposes and to help verify the accuracy of your
tax return. The IRS may also provide this information to the Department of
Justice for civil and criminal litigation and to cities, states, and the
District of Columbia to carry out their tax laws. You must provide your taxpayer
identification number whether or not you are required to file a tax return.
Payers must generally withhold up to 30% of taxable interest, dividends and
certain other payments to a payee who does not furnish a taxpayer identification
number to a payer. Certain penalties may also apply.

                                   ----------

                                       6

<PAGE>

             For additional information, contact your tax consultant
                        or the Internal Revenue Service.

                                       7