NiO/NaTaO3:La photocatalyst was prepared by solid-state reaction and impregnation methods. We investigated the photocatalytic overall water splitting on NaTaO3-based photocatalysts by in situ ATR-IR spectroscopy. Two new species with IR absorption peaks centered at 1350 and 870 cm-1 were observed and assigned to H2O2 species. H2O2 is formed by protonation of Ta(O2), a water oxidation intermediate, when it is surrounded by a large amount of water.