The electrocatalytic reduction of oxygen was carried out at Nafion(R)- and clay-modified electrodes in the presence of tetraazamacrocyclic cobalt(III) complexes. Long-time electrolysis experiments were performed, and the faradaic yield of H2O2 and the rate of H2O2 production were found to be higher at clay-coated electrodes than at Nafion(R)-coated electrodes. In the case of the photoelectrocatalytic reduction of oxygen at Pt/Nafion(R)/[Ru(bpy)(3)](2+) and Pt/H+-montmorillonite/[Ru(bpy)3](2+) electrodes, the amounts of H2O2 produced increased by a factor of two under light irradiation condition when compared to dark. The faradaic yield of H2O2 under irradiation conditions was determined at the modified electrodes, and a mechanism for multistep one-electron photoelectrocatalytic reduction of oxygen was proposed.