Exhibit 31.2CERTIFICATIONSI, William Garrahan, certify that:1. I have reviewed this quarterly report on Form 10-Q of XO Communications, Inc.;2. Based on my knowledge, this quarterly report does not contain any untrue statement of a materialfact or omit to state a material fact necessary to make statements made, in light of thecircumstances under which such statements were made, not misleading with respect to the periodcovered by this report;3. Based on my knowledge, the financial statements, and other financial information included inthis quarterly report, fairly present in all material respects the financial condition, results ofoperations and cash flows of the registrant as of, and for, the periods presented in this quarterlyreport;4. The registrant’s other certifying officer and I are responsible for establishing and maintainingdisclosure controls and procedures (as defined in Exchange Act Rules 13a-15(e) and 15d-15(e)) andinternal control over financial reporting (as defined in the Exchange Act Rules 13a-15(f) and15d-15(f)) for the registrant and have: a) designed such disclosure controls and procedures, or caused such disclosure controlsand procedures to be designed under our supervision, to ensure that material informationrelating to the registrant, including its consolidated subsidiaries, is made known to us byothers within those entities, particularly during the period in which this quarterly reportis being prepared;  b) designed such internal control over financial reporting, or caused such internalcontrol over financial reporting to be designed under our supervision, to providereasonable assurance regarding the reliability of financial reporting and the preparationof financial statements for external purposes in accordance with generally acceptedaccounting principles;  c) evaluated the effectiveness of the registrant’s disclosure controls and procedures andpresented in this quarterly report our conclusions about the effectiveness of thedisclosure controls and procedures, as of the end of the period covered by this reportbased on such evaluation; and  d) disclosed in this report any change in the registrant’s internal control over financialreporting that occurred during the registrant’s most recent fiscal quarter that hasmaterially affected, or is reasonably likely to materially affect, the registrant’sinternal control over financial reporting; and5. The registrant’s other certifying officer and I have disclosed, based on our most recentevaluation of internal control over financial reporting, to the registrant’s auditors and the auditcommittee of registrant’s board of directors (or persons performing the equivalent function): a) all significant deficiencies and material weaknesses in the design or operation ofinternal control over financial reporting which are reasonably likely to adversely affectthe registrant’s ability to record, process, summarize and report financial information;and  b) any fraud, whether or not material, that involves management or other employees whohave a significant role in the registrant’s internal controls over financial reporting.Date: August 9, 2005                                                                           /s/ William Garrahan   William Garrahan   Senior Vice President and