It is Found that carbon monoxide (CO) poisoning Could be mitigated by increasing only cathode back-pressure for a proton exchange membrane fuel cell (PEMFC) With ultra-thin membranes (<= 25 mu m). This mitigation call be explained by a heterogeneous oxidation of CO oil a Pt-Ru/C anode by the permeated O-2 which is known as "internal air bleed" in his paper. A steady-state model which accounts for this internal air bleed has been developed to model the Pt-Ru/C anode polarization data when 50 ppm CO in H-2 is used as anode feed gas. The modeling results show that the mitigation of CO poisoning by the internal air bleed even exists at ambient conditions for a PEMFC with all Ultra-thin membrane. Therefore. the effect of internal air bleed must be considered for modeling fuel cell performance or anode polarization data if all ultra-thin membrane and a low level of CO concentration are used for a Pt-Ru/C anode. An empirical relationship between the amount of internal air bleed used for the mitigation of CO poisoning and the fraction of free Pt sites is provided to facilitate the inclusion of an internal air bleed term in the modeling Of anode polarization and the fuel cell performance. (C) 2009 Elsevier B.V. All rights reserved.