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Exhibit 12.1 Certification of Chief Executive Officer I, René Sagastuy, certify that: (i)   I have reviewed
this annual report on Form 20-F of Maxcom Telecomunicaciones, S.A. de C.V.; (ii)   Based on my
knowledge, this annual 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 annual
report; (iii)   Based on my
knowledge, the financial statements, and other financial
information included in this annual 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 annual report; (iv)   The companys
other certifying officer 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)) and internal control over financial
reporting (as defined in Exchange Act Rules 13a-15(f) and 15d-15(f)) for the
company and have: 1.     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 annual report is being prepared; 2.     designed such internal control over financial
reporting, or caused such internal control over financial reporting to be
designed under our supervision, to provide reasonable assurance regarding
the reliability of financial reporting and the preparation of financial
statements for external purposes in accordance with generally accepted accounting principles; 3.     evaluated
the effectiveness of the companys disclosure controls and procedures
and presented in this annual report our conclusions about the effectiveness
of the disclosure controls and procedures as of the period covered by
this report based on such evaluation; and 4.     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 (v)   The companys
other certifying officer 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 companys board of directors (or
persons performing the equivalent function): 1.     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 2.     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. /s/ RENE SAGASTUYRené SagastuyChief Executive Officer Dated: June 29, 2005