The electrochemical behavior and the electrocatalytic activity of ruthenium electrode for oxygen reduction are examined using cyclic voltammetry and rotating ring-disk electrode techniques. A surface oxide film on ruthenium metal is formed when the potential is scanned in the positive direction of hydrogen adsorption and evolution region. This oxide him affects the activity of ruthenium for oxygen reduction reaction. Oxygen reduction on ruthenium electrode proceeds primarily by a direct 4-electron reduction pathway without producing significant amounts of solution phase peroxide. The kinetic results suggest the initial electron transfer as the rate-determining step. (C) 2000 Elsevier Science Ltd. All rights reserved.