Electrooxidation kinetics of mixtures of carbon monoxide and hydrogen were studied on well-characterized surfaces of Pt, Ru, and two Pt-Ru alloys (Ru surface composition of approximate to 50 atomic % and approximate to 90 atomic %, respectively, determined by low energy ion scattering) in 0.5 M H2SO4 at 62 degrees C. Subsequent to electrode cleaning in UHV by ion-sputtering, the electrodes were transferred from UHV into a rotating disk electrode (RDE) configuration in a conventional electrochemical cell. While pure Ru is completely inactive for CO/H-2 electrooxidation, CO/H-2 oxidation on the alloy electrodes is characterized by two states of activity : a state of low but finite activity at potentials below ca. 0.3-0.4 V, and a highly active (mass-transfer limited) state at more positive potentials. The exact potential of the transition shifts negatively with an increase in the Ru surface composition, and a decrease in the CO concentration (from 2% to 0.1% CO), consistent with the measured negative reaction order with respect to solution phase CO. In contrast, pure Pt surfaces have no measurable activity below 0.4 V and the transition to a highly active state occurs at potentials above 0.5-0.7 V. On Pt and Pt-Ru alloys, the transition to the highly active state coincides with the oxidative stripping of adsorbed CO, while the reaction at potentials below 0.4 V on the alloy surfaces appears to be the oxidation of hydrogen via transient holes in the CO adlayer. The absolute activity of the Pt-Ru alloys at potentials below the transition potential is too low to be of practical use in polymer electrolyte membrane (PEM) fuel cells; in such cells the anode overpotential with CO/H-2 mixtures of 0.1-2% CO will be at least 0.35 V.