The electrooxidation of pure hydrogen and carbon monoxide on Pt-Ru alloy surfaces was studied as a prelude to a companion study (Part 2) of the electrooxidation of CO/H-2 mixtures such as those produced by steam reforming of hydrocarbon fuels. The rotating disk electrode (RDE) technique was uniquely combined with characterization of the electrode surface composition in UHV by low-energy ion-scattering spectrosocopy (LEIS). Whereas hydrogen oxidation was found to be a very fast reaction on both pure Pt and Pt-rich Ru ahoy surfaces (an exchange current density on the order of 0.1 A/cm(2)), it is orders of magnitude slower on a pure Ru surface at room temperature. However, unlike pure Pt, the reaction is thermally activated on pure Ru, such that at 60 degrees C the reaction occurs at both Pt and Ru sites of the alloy surface at practical rates. The onset potential for CO oxidation is inversely related to the Ru content of the surface and is the lowest for pure Ru. However, the maximum current density, equal to the diffusion-limiting current, occurs on the 50% alloy surface. These results are consistent with the previously proposed mechanism for the oxidation of adsorbed CO on these same surfaces when the effect of a continuous flux of CO from the electrolyte is accounted for. According to this mechanism, Ru sites nucleate oxygenated species at low potential, but the species formed on Pt/Ru site pails are uniquely reactive toward adsorbed CO. Parallels are drawn between H-2 and CO oxidation and methanol and formic acid oxidation on these surfaces, as all four reactions have some elementary steps in common.