The oxidation kinetics of H-2 and H-2 + 100 ppm CO were investigated on Pt, Ru and Pt-Ru electrocatalysts supported on a high-surface area carbon powder. The atomic ratios of Pt to Ru were 3, 1 and 0.33. XRD, TEM, EDS and XPS were used to characterize the electrocatalysts. When alloyed with ruthenium, a decrease in mean particle size and a modification of the platinum electronic structure were identified. Impedance measurements in H2SO4, at open circuit potential, indicated different mechanisms for hydrogen oxidation on Pt/C (Tafel-Volmer path) and Pt-Ru/C (Heyrowsky-Volmer path). These mechanisms also occur in the presence of CO. Best performances, both in H-2 and H-2 + CO, were achieved by the catalyst with the ratio Pt/Ru = 1. This is due to a compromise between the number of free sites and the presence of adsorbed water on the catalyst. For CO tolerance, an intrinsic mechanism not involving CO electroxidation was proposed. This mechanism derives from changes in the electronic structure of platinum when alloyed with ruthenium.