The electrochemical oxidation of hydrogen (H-2), carbon monoxide (CO), and their mixtures (500 ppm-2%) on a well-characterized Pt70Mo30 bulk alloy was examined using the rotating disk electrode technique in 0.5 M H2SO4 at 333 K, The electrodes were transferred to and from a UHV chamber, where surface analyses were conducted using a combination of low-energy ion scattering (LEIS), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). The surface composition of this alloy after sputter-etching and annealing in UHV was slightly enriched in Pt to a composition of Pt77Mo23 The kinetics of H-2 oxidation are not measurably affected by the presence of the Mo in the surface. The shapes of the polarization curves for the oxidation of the H-2/CO mixtures are qualitatively similar to those for the Pt50Ru50 alloy examined previously : a high current-potential slope ica. 0.5 V/dec at low overpotential followed by a transition to a highly active state when the current approaches the diffusion-limiting current; the potential where the transition to the active state occurs decreases with decreasing CO concentration and decreases with increasing temperature; the current in the low-overpotential region is roughly inverse half-order in the CO partial pressure. The current densities in the low-overpotential region an comparable to those on Pt50Ru50 alloy and about a factor Or 50 times those on a pure rt surface, while the anodic stripping of irreversibly adsorbed CO is very different from that on the Pt50Ru50 alloy, with most of the COads being oxidized only above 0.6 V (RHE), there is direct evidence for the oxidation of small amounts of COads in the low-overpotential region of 0.05-0.5 V. The mechanism of action of Mo in the Pt surface in enhancing Hr oxidation in the presence of CO thus appears to be very similar to that of Ru : a reduction in the steady-state coverage of COads on the Pt sites by oxidative removal, freeing Pt sites for H-2 oxidation.