Gas-solid heterogeneous photooxidation of toluene over TiO 2 catalyst was studied to investigate the factors controlling the catalytic activities. The toluene photooxidation behavior on TiO 2 was strongly affected by the formation and oxidation behavior of intermediate compounds on TiO 2 , and their accumulation decreased the reaction rate for toluene photooxidation. The formation and oxidation behavior of the byproduct compounds depended on the initial concentration of toluene and water vapor. In situ Fourier transform infrared (FTIR) studies revealed that water vapor promoted the cleavage of the aromatic ring and facilitated CO 2 formation. At the reaction temperature of 300 K, the deposition of Pt on TiO 2 suppressed CO formation, whereas catalytic activity was decreased due to the increase in the amount of intermediate compounds. On the other hand, Pt/TiO 2 showed higher activity than TiO 2 at 353 K, in spite of the increase of the intermediate compounds.