The porous oxynitride TaON film electrode prepared on conducting glass (FTO) showed significantly high quantum efficiency (IPCE = ca. 76% at 400 nm at 0.6 V vs Ag/AgCl) in an aqueous Na(2)SO(4) solution, after loading of IrO(2) x nH(2)O nanoparticles as a cocatalyst for water oxidation. Overall water splitting into H(2) and O(2) under visible light was demonstrated using an IrO(2) x nH(2)O-loaded TaON photoanode combined with a Pt electrode under an externally applied bias (0.6-1 V).