Electrocatalytic O-2 reduction was studied using a modified electrode coated with a Nafion(R) (Nf) membrane dispersing an osmium complex ([Os-III(bpy)(2)Cl-2]Cl). When measuring cyclic voltammogram (CV) under O-2 atmosphere, growth of cathodic currents due to the O-2 reduction was found to take place on reducing Os-III into Os-II (below 0 V (versus Ag/AgCl)). Although a similar CV was also measured in acetonitrile containing the Os complex, no catalysis for O-2 reduction by the Os complex was observed on voltammetric aspect. Therefore, it appeared that the Os complexes aggregated in the polymer matrix are responsible for the electrocatalysis involving a multi-electron transfer. A selective and direct catalysis for O-2 reduction into H2O was suggested from a ring-disk voltammogram to take place by the present system. In addition, the dependencies of both the catalytic currents for O-2 reduction (-I-cat) and the apparent diffusion coefficient of electron (D-app) on the complex concentration were investigated. Based on those results, the factor that affects the overall kinetics in the present catalysis system was discussed. (C) 2003 Elsevier B.V. All rights reserved.