Oxygen reduction kinetics on bare and Pt-modified Ru(10 (1) over bar0) and Ru(0001) single crystal surfaces, and on carbon-supported Ru nanoparticles have been investigated. Spontaneous deposition of Pt was used to form approximately 1.5 and 0.5 monolayers on Ru single crystals and nanoparticles, respectively. The reaction kinetics of O-2 on single crystal surfaces has a small structural dependence. It is also affected by the oxidation state of the Ru surfaces. The reaction involves the exchange of approximately four electrons per O-2 molecule, with the transfer of the first electron being the rate determining step. A deposit of 1.5 monolayers of Pt makes the surfaces considerably more active than bare Ru, but nevertheless still less active than bulk Pt. An electrocatalyst made by the deposition 0.5 monolayers of Pt on carbon-supported Ru nanoparticles is somewhat less active than commercial catalysts, but contains considerably lower Pt loadings.