The electrochemical oxidation of hydrogen in the presence of carbon monoxide (0.05-2%) on a well-characterized Pt75Mo25 alloy surface was examined using the rotating disk electrode technique in 0.5 M H2SO4 at 333 K. The surface composition of this alloy determined by low-energy ion scattering after sputter cleaning and annealing in UHV was essentially the same as the bulk. The shapes of the polarization curves are qualitatively similar to those for the Pt50Ru50 alloy examined previously : a high Tafel-slope (ca. 0.5 V/dec) region at low overpotential followed by a transition to a highly actively state where the current approaches the diffusion-limiting current; the potential where the transition to the active state occurs decreases with decreasing CO concentration, and the current in the low-overpotential region is roughly inverse half-order in the CO partial pressure. The magnitude of the current in the low-overpotential region on the Pt75Mo25 alloy is nearly the same as on the Pt50Ru50 alloy, but the potential for the transition to the active state is about 0.15 V higher. The magnitude of the current at 0.05-0.1 V with H-2 containing 100 ppm CO is sufficiently high that Pt-Mo alloy is of technical interest as an anode catalyst for low-temperature fuel cells fed with a reformed hydrocarbon fuel.