Abstract The kinetics of oxygen reduction have been studied on a ruthenium electrode in alkaline solutions using the rotating disc and rotating disc-ring methods. The reaction kinetics and mechanism were found to depend on the oxidation state of the ruthenium surface. For a surface having up to two monolayers of oxide a “parallel” mechanism of O 2 reduction has been found, which goes predominantly through a four-electron direct reduction to OH −. All the rate constants of the O 2 and HO − 2 reactions, including those of the adsorption equilibrium of HO − 2 on the Ru surface, have been determined. No catalytic decomposition of HO − 2 was found. Although the Tafel slope is higher than −120 mV/dec it appears that an initial electron exchange is the rate-determining step. The kinetics are slower on a Ru surface with a thicker oxide layer. The mechanism of the reaction is also different, showing a potential-dependent ratio of the rate constants for a direct and series reduction of O 2 and a catalytic decomposition of HO − 2.