The porous oxynitride TaON film electrode prepared on conducting glass (PTO) showed significantly high quantum efficiency (IPCE = ca. 76% at 400 nm at 0.6 V vs Ag/AgCl) in an aqueous Na2SO4 solution, after loading of IrO2 center dot H2O nanoparticles as a cocatalyst for water oxidation. Overall water splitting into H-2 and O-2 under visible light was demonstrated using an IrO2 center dot nH(2)O-loaded TaON photoanode combined with a Pt electrode under an externally applied bias (0.6-1 V).