Photocatalysis of Cu2O powder was investigated to evaluate its ability for the stoichiometric decomposition of water into O-2 and H-2. Although thus far it has been indicated that Cu2O can function as a photocatalyst for the overall water splitting, H-2 evolution was found to take place solely while there was no generation of O-2. Moreover, the evolution rate of H-2 gradually decreased during prolonged irradiation, and consequently, the evolution of H-2 ceased. From the structural analyses of deactivated Cu2O, it appeared that the H-2 evolution originates in the self-oxidation of Cu2O to CuO. (C) 2008 The Electrochemical Society. [DOI: 10.1149/1.3054330] All rights reserved.