The electrocatalytic activity of well-characterized Pt-Ru alloy electrodes toward the electrooxidation of CO in acidic electrolyte at room temperature was measured on alloy surfaces prepared in UHV (ultrahigh vacuum). Clearly defined surface composition was determined via LEIS (low-energy ion scattering). Electrocatalytic activities were measured by CO stripping voltammetry as well as by potentiostatic oxidation of adsorbed CO. It was found that the property of Ru atoms to nucleate oxygen-containing species at low potentials produced a strong enhancement in the catalytic activity of sputter-cleaned Pt-Ru alloy electrodes compared to pure Pt, thereby supporting the concept of the bifunctional character of the oxidation process of these alloys. A further synergistic effect of the alloy with a Ru surface composition of approximate to 50 atom % Ru was observed, with a catalytic shift in the CO electrooxidation current of -0.25 and -0.15 V compared to those of pure Pt and pure Ru surfaces, respectively. This synergism was attributed to a uniquely active state of OHads on Pt-Ru pair sites. The different electrocatalytic activities of sputter-cleaned versus annealed Pt-Ru alloy electrodes with essentially identical Ru surface compositions are discussed in terms of Ru clustering during annealing.