The electrocatalysis of CO tolerance in the hydrogen oxidation reaction was investigated for Pt/C, PtSn/C and PtRu/C electrocatalysts in proton exchange membrane (PEM) fuel cells. Both half and single cell polarization characteristics were studied at several temperatures and CO partial pressures. It is proposed that the CO adsorption step occurs predominantly through a displacement path for PtSn/C and through a free site attack path for CO on both Pt/C and PtRu/C. The data are more consistent with the participation of linear (PtSn/C, and PtRu/C [T less than or equal to 55 degrees C]) and bridged bonded adsorbed CO on Pt/C and PtRu/C [T greater than or equal to 70 degrees C]. The CO oxidation process occurs at different potentials depending on the nature of the electrode material. The oxidation of CO by the alloy catalysts is not the only contributor to CO tolerance. Changes in the thermodynamics and the kinetics of the CO adsorption process, induced by the alloy catalysts, also contribute to CO tolerance.