The mechanism of water photooxidation (oxygyen photoevolution) on a TaON photocatalyst was studied on the basis of our previous studies on the mechanism of this reaction on TiO2, and N-doped TiO2. We have confirmed that photocatalytic O-2 evolution occurs from an aqueous TaON suspension in the presence of Fe3+, as reported. In-situ MIR-IR experiments have indicated that the TaON surface is slightly oxidized under visible-light irradiation, indicating that the oxygen photoevolution on TaON actually occurs on a thin Ta-oxide overlayer. The in-situ MIR-IR experiments have also shown that a certain surface peroxo species, tentatively assigned to adsorbed HOOH, is formed as an intermediate of the O-2 photoevolution reaction. Studies on the effect of addition of reductants to the electrolyte on the IPCE have shown that photogenerated holes at the TaON surface cannot oxidize reductants such as SCN-, Br-, methanol, ethanol, 2-propanol, and acetic acid, though they can oxidize H2O into O-2. Detailed considerations of these results have strongly suggested that the water photooxidation reaction on TaON proceeds by our recently proposed new mechanism, that is, the reaction is initiated by a nucleophilic attack of a water molecule (Lewis base) on a surface-trapped hole (Lewis acid).