The CO tolerance mechanism of the hydrogen oxidation reaction was investigated on several highly dispersed carbon-supported nanocrystalline Pt and binary Pt alloys. For this purpose, current/potential behavior was derived from half-cells under actual proton exchange membrane fuel cell operating conditions and correlated with expressions derived from kinetic models. Kinetic analyses have shown that the CO poisoning effect on Pt/C, PtRu/C, and PtSn/C catalysts occurs through a free Pt site attack mechanism, involving bridge- and linear-bonded adsorbed CO. For all catalysts, the onset of CO oxidation occurs via the bridge- bonded species, but for PtRu/C and PtSn/C, the reaction starts at smaller potentials. Under this condition, the hydrogen oxidation currents are generated on the vacancies of a carbon monoxide adsorbed layer created when some of the bridge- bonded CO molecules are oxidized. The linearly adsorbed CO is oxidized at higher overpotentials, leading to an increase of the holes on the CO layer and thus of the rate of the hydrogen oxidation process.