Tungsten oxides (WO₃) loaded with platinum (Pt) nanoparticles exhibited high activity for the decomposition of organic compounds in the presence of molecular oxygen under visible light irradiation. An action spectrum analysis for decomposition of acetic acid in an aqueous solution has shown that the reaction over the Pt-WO₃ can be ascribed to the band-gap photoexcitation of WO₃ utilizing visible light up to 470 nm, and the apparent quantum efficiency in visible light region was much higher than that of the titania (TiO₂)-based visible light-driven photocatalysts such as sulfur-doped TiO₂. Transient photoabsorption analysis using photoacoustic (PA) spectroscopy indicated that the photoexcited electrons in WO₃ reacted with molecular oxygen in spite of more positive conduction band level of WO₃ compared to the potential of oxygen reduction, and the rate of reaction was significantly increased by Pt loading.