The effect of H2O vapor addition on the photocatalytic oxidation of ethanol, acetaldehyde and acetic acid on TiO2 semiconductor powders under UV light irradiation were investigated. The photocatalytic oxidation of acetaldehyde and acetic acid on TiO2 surfaces was enhanced by the addition of H2O vapor, its extent depending on the pressure while, on the other hand, the photocatalytic oxidation of ethanol was depressed by its coexistence with the H2O vapor. Since the Ti4+ sites on the TiO2 surfaces work as trap sites for the photo-formed electrons, the carbonyl compounds strongly adsorbed on the Ti4+ sites are not directly oxidized by the holes. However, when optimal amounts of H2O molecules exist in the reaction system, the OH radicals formed by the photo-oxidation of H2O molecules indirectly oxidize such organic species on the Ti4+ sites. In contrast, although ethanol molecules promptly interact with the photo-formed holes on the hydroxyl groups, the coexistence of H2O molecules in the reaction system lessens the efficient interaction between ethanol and the hydroxyl groups on TiO2 surfaces, resulting in a suppression of photocatalytic reactivity. In this way, the correlation of the different surface sites, such as Ti4+ or the hydroxyl groups, on which organic compounds adsorb and the photo-formed carriers are trapped, is not clearly understood but is observed to play an important role in the efficient photocatalytic oxidation of various organic compounds on TiO2 semiconductor in the presence or the absence of an H2O vapor. (C) 2010 Elsevier B.V. All rights reserved.