The composite film of Nation and the conducting polymer was prepared by dropping Nafion solution containing dispersed powder of the conducting polymer on a substrate, followed by drying. The conducting polymers used in this study were polypyrrole and polyanitine. Irradiation of the film immersed in acidic solution containing chloroplatinate and ascorbate with a 500 W xenon lamp (lambda > 360 nm) induced Pt deposition on the composite film surface. Fluorescent X-ray analysis has revealed that amount of deposited Pt increased with irradiation time and that deposition rate on the Nafion-polyaniline composite was much higher than that on the Nation-polypyrrole composite. Electrochemical measurements of the composite film-coated glassy carbon electrodes with and without deposited Pt have been conducted to examine their activities toward O-2 reduction. Voltammetry measurements showed definite cathodic currents due to O-2 reduction on deposited Pt particles. The Pt-deposited polyaniline composite possessed much higher catalytic activities than Pt-deposited polypyrrole composite because of difference in Pt amount. In order to elucidate some parameters that influence the catalytic activities, O-2 reduction experiments were made by using the Nafion-polyaniline composite films prepared under different conditions including total amount of the composite film and amount ratio of polyaniline to Nation. (C) 2004 Elsevier Ltd. All rights reserved.