Electrocatalysis of anode electrode tolerance resulting from the presence of both CO and CO2 in the reformer feed was investigated for Pt, Pt-Ru (1:1) and various atomic ratios of supported Pt:Mo electrocatalysts in proton exchange membrane fuel cells (PEMFCs). In order to elucidate the effects of CO and CO2 in the reformer feed, separate systematic studies were conducted with varying levels of CO and CO2 in H-2. The results were used to explain those obtained with a fixed reformate composition: 45% H-2, 10 ppm CO, 15% CO2, 1% CH4 balanced with N-2. Results with CO in H-2 showed that PtMo/C exhibits at least a threefold better CO tolerance as compared to PtRu/C and fourfold with respect to Pt/C. The variation of PtMo atomic composition has a negligible effect on CO tolerance. Additional surface poisoning was detected for all the electrocatalysts studied in the molar ratio (H-2:CO2, 40:60 to 60:40). The presence of a reduced CO2 Species was confirmed using cyclic voltammetry. An ensemble effect was proposed to explain the variation of tolerance to CO2 as a function of Pt: Mo atomic ratio, this is in contrast to the effect in the presence of adsorbed CO. Interestingly, the overpotential losses in the presence of H-2:CO2 for PtMo/C (1: 1) and PtRu/C (1: 1) were very close. As the Pt content of the PtMo/C alloys was increased, the overpotential losses followed those observed for pure Pt, clearly demonstrating a relationship between overpotential loss and Pt site availability. Despite similar overpotential losses between Pt/C and PtMo/C (5: 1), both of which were greater than PtRu/C (1; 1) the overpotential loss observed for PtMo in a CO2 + CO reformate mix was far better than for both PtRu/C and Pt/C. (C) 2003 Elsevier B.V. All rights reserved.