Photocatalytic oxidation of organic adsorbates on anatase TiO2 films has been examined in different atmospheres of humid air, dry air and vacuum. The photocatalytic oxidation was observed by IR absorption spectroscopy (IRAS) with a multiple-internal-reflection (MIR) geometry. The photocatalytic oxidation is the fastest in the air at a humidity of 70% where oxygen and water vapor are consuming to produce OH radicals and O-2(-) anions on the TiO2 surface with the UV exposure. In the dry air, a rate of the photocatalytic oxidation is almost 30% of that in the humid air, where only O-2(-) anions oxidize the organic adsorbates. In vacuum, on the other hand, it is negligible, which suggests that adsorbed H2O molecules do not play an important role in the photocatalytic reaction. It is suggested that an addition of the water vapor is necessary to achieve the higher catalytic activity. (c) 2006 Elsevier B.V. All rights reserved.