The kinetics of the electrochemical oxidation of carbon monoxide (CO) and CO/hydrogen mixtures (0.1 and 2% CO) in sulfuric acid electrolyte at 25-62 degrees C was studied on different surfaces of the ordered single crystal Pt3Sn alloy. Characterization of the surface composition and structure was determined in UHV using low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and low energy ion scattering (LEIS) prior to determining the electrode kinetics using the classical rotating disk method (RDE) with CO dissolved in the electrolyte. Clean annealed and sputtered-cleaned but not-annealed surfaces of (110) and (111) orientation were studied. A remarkable difference in activity was observed between the annealed(lll) surface and the sputtered but not-annealed (110) surface, with both surfaces having the same nominal surface composition, 20-25 at% Sn, but different local structures. The onset potential for CO oxidation on the (111) surface was shifted cathodically by 0.13 V relative to that for the sputtered (110) surface, and the onset comes remarkably close to 0 V on the reversible hydrogen potential scale. Relative to pure Pt surfaces (of any crystal structure), the potential shift is more than 0.5 V, corresponding to a catalytic activity that is higher by more than four orders of magnitude. Comparable shifts were observed for the oxidation of CO/H-2 mixtures. Both the structure sensitivity and the high catalytic activity of the Pt3Sn surface are attributed to an adsorbed state of CO unique to this alloy and occurs at relatively high coverage on the (111) surface.