To develop a new cathode without using Pt for a H-2/O-2 polymer-electrolyte-membrane-fuel-cell, we studied the possibility of using a Cu/ carbon cathode for the reduction of O-2. The plan for development of the Cu/carbon cathode is: (i) formation of redox functional groups on carbon to promote electron-transfer reaction, (ii) deposition of phosphorous groups on carbon to promote proton diffusion and (iii) loading Cu on the modified carbon support. The electro-catalytic activity of the Cu/carbon cathode was not so excellent as that of the Pt/carbon cathode, but it was fairly good at lower P(O-2). To clarify the Cu function for the acceleration of the O-2 reduction, we characterized the Cu/carbon electro-catalyst with XRD, SEM and CV measurements. When the oxidation state of Cu was 2+ at higher cell voltages, the reduction of O-2 was accelerated. On the other hand, when metallic Cu was formed at lower cell voltages, the enhancing effect of Cu disappeared. The CV data strongly suggested that Cu2+ species functioned as an adsorption site of O-2, not as a redox mediator. On the basis of the experimental results, a suitable model of the reduction mechanism of O-2 over the Cu/carbon cathode was proposed. (C) 2004 Elsevier B.V. All rights reserved.