Photocatalytically active thin films of TiO2 on transparent supports display different activities depending on the direction of illumination in relation to the reactant gas. In the case, where illumination and reactant gas are both supplied from the same side, the activity goes toward an asymptotic value for increasing catalyst film thickness. Conversely, having opposing directions of illumination and incident gas, there exists an optimal catalyst film thickness with respect to activity. In the present work, we demonstrate, quantify and model this effect using methane photooxidation over PVD TiO2 thin films and derive analytical solutions to the gas diffusion/reaction system. The analytical model is fitted to the data to estimate relevant kinetic parameters such as gas diffusivity and reaction rate constants. The activity of samples is predicted quantitatively by the model, regardless of wavelength and intensity of illumination. The model serves as a general method to predict activities of photocatalytic films on transparent supports. (C) 2012 Elsevier Inc. All rights reserved.