Abstract Electrocatalytic O 2 reduction was studied using a modified electrode coated with a Nafion membrane (Nf) dispersing a trinuclear ruthenium ammine complex ([(NH 3) 5Ru IIIORu IV(NH 3) 4ORu III(NH 3) 5]Cl 6, Ru-red). When measuring cyclic voltammogram under O 2 atmosphere (at 0.5 mV s −1), catalytic currents due to O 2 reduction were found to develop below −0.2 V (vs. Ag/AgCl). Since Ru-red undergoes irreversible decomposition into the mononuclear complexes via the reduced state (Ru III–Ru III–Ru III) (∼−0.1 V), it is suggested that the electrocatalysis originates from the decomposed species (initial active species: Ru II(NH 3) 5(OH 2) and Ru II(NH 3) 4(OH 2) 2) rather than from the Ru-red. Although the present electrocatalyst was also applied to H 2O 2 reduction system, the catalytic activity was found to be poor from the voltammetric behavior. It appeared that the kinetics of the electrocatalysis is much faster in the O 2 reduction than in the H 2O 2 one. A selective and direct catalysis for O 2 reduction into H 2O was suggested from a ring–disk voltammogram to take place by an aggregate of the mononuclear ruthenium complexes in the polymer matrix. In addition, it was found that electrocatalytic O 2 reduction involves a slow kinetic process, so that factors affecting the overall kinetics were discussed in terms of the catalysis mechanism.