The electrochemical oxidation of hydrogen, carbon monoxide, and dilute mixtures of carbon monoxide in hydrogen on a well-characterized Pt3Sn alloy electrode with a Sn surface concentration of approximate to 20 at.% was examined using the rotating disk electrode technique in 0.5 M H2SO4 at 62 degrees C. The potential for the onset of carbon monoxide oxidation on this surface was nearly 0.5 V lower than for pure Pt and approximately 0.15 V lower than for pure Ru or any Pt-Ru alloy and represents the lowest overpotential for CO oxidation ever seen in this laboratory. The potential for the onset of the oxidation of hydrogen in the presence of 0.1-2% CO coincides with the potential for the onset of CO oxidation on the alloy surface. The polarization curve for the oxidation of these gas mixtures is shifted cathodically by approximate to 0.35 V with respect to that for pure Pt.