<PAGE>
                                                                    Exhibit 12.1


                                  Certification

I, Alberto Guimaraes, certify that:

     1.   I have reviewed this annual report on Form 20-F of Petrobras Energia
          Participaciones, S.A.;

     2.  Based on my knowledge, this report does not contain any untrue
         statement of a material fact or omit to state a material fact necessary
         to make the statements made, in light of the circumstances under which
         such statements were made, not misleading with respect to the period
         covered by this report;

     3.  Based on my knowledge, the financial statements, and other financial
         information included in this report, fairly present in all material
         respects the financial condition, results of operations and cash flows
         of the company as of, and for, the periods presented in this report;

     4.  The company's other certifying officer(s) and I are responsible for
         establishing and maintaining disclosure controls and procedures (as
         defined in Exchange Act Rules 13a-15(e) and 15d-15(e)) for the company
         and have:

                  (a) Designed such disclosure controls and procedures, or
                  caused such disclosure controls and procedures to be designed
                  under our supervision, to ensure that material information
                  relating to the company, including its consolidated
                  subsidiaries, is made known to us by others within those
                  entities, particularly during the period in which this report
                  is being prepared;

                  (b) Evaluated the effectiveness of the company's disclosure
                  controls and procedures and presented in this report our
                  conclusions about the effectiveness of the disclosure controls
                  and procedures, as of the end of the period covered by this
                  report based on such evaluation; and

                  (c) Disclosed in this report any change in the company's
                  internal control over financial reporting that occurred during
                  the period covered by the annual report that has materially
                  affected, or is reasonably likely to materially affect, the
                  company's internal control over financial reporting; and

     5.  The company's other certifying officer(s) and I have disclosed, based
         on our most recent evaluation of internal control over financial
         reporting, to the company's auditors and the audit committee of the
         company's board of directors (or persons performing the equivalent
         functions):

                  (a) All significant deficiencies and material weaknesses in
                  the design or operation of internal control over financial
                  reporting which are reasonably likely to adversely affect the
                  company's ability to record, process, summarize and report
                  financial information; and
<PAGE>
                  (b) Any fraud, whether or not material, that involves
                  management or other employees who have a significant role in
                  the company's internal control over financial reporting.


Date: June 30, 2004

                             /s/ Alberto Guimaraes
                             -----------------------
                             Alberto Guimaraes
                             Chief Executive Officer



                                       2