Photocatalytic oxidation of two ketones having different hydrophilicity, acetone and methyl isobutyl ketone (MIBK), over TiO2 thin films supported on glass rings was studied under different relative humidity (RH) conditions. Adsorption isotherms of both ketones in the dark under different concentrations of water vapor were also measured and modeled considering a two-site Langmuir model. As expected, an increase in RH resulted in a decrease in the surface concentration of the organic molecules, but the photocatalytic oxidation rates did not show a parallel variation. In the case of acetone, almost total mineralization was achieved, and the kinetic constants obtained from fitting the data to the Langmuir-Hinselwood-Hougen-Watson (LHHW) model, were significantly larger in the presence of water vapor. Photocatalytic elimination of methyl isobutyl ketone (MIBK) is proposed to take place simultaneously by a direct oxidation route and by a sequential mechanism. Acetone is a stable intermediate in this last reaction pathway. Photodegradation of MIBK at different levels of RH was also modeled using LHHW kinetic expressions, and the rates obtained were slightly lower than those obtained for the photooxidation of acetone. However, a carbon balance was not achieved for MIBK, very likely due to the accumulation of artially oxidized products on the surface of the photocatalyst. The influence of water vapor on the photocatalytic oxidation rate of this larger ketone is more complex. and both inhibition and promotion effects can be envisaged. In addition, experimental results show that humidity exerts a significant influence in the mineralization efficiency of MIBK. (C) 2003 Elsevier Science B.V. All rights reserved.