The effect of oxygen concentration on the photocatalytic degradation rate of oxalic acid on a fixed layer of TiO2 particles in a batch mode plate photoreactor was investigated at various light intensities. The regions where the photocatalytical decomposition rate is controlled by the flux of oxygen, photons, or both, were identified. For low oxygen concentration (0-0.15 mol m(-3)) and photon flux intensity in the range from 10 to 24 x 10(-5) einstein m(-2) s(-1) the experimentally determined photocatalytical decomposition rate was in agreement with that theoretically calculated assuming the process to be controlled by the limiting flux of oxygen to the TiO2 surface. At higher concentrations of oxygen (0.15-0.94 mol m(-3)) the rate of photocatalysis was controlled simultaneously by both the flux of oxygen and photons. The influence of the oxygen concentration decreased with decreasing photon flux. For low photon flux intensities (less than or equal to 3.5 x 10(-5) einstein m(-2) s(-1)), the reaction rate was controlled by the photon flux. The concentration profile of oxygen in the diffusion layer along the reactor plate was calculated and showed a significant decrease in oxygen concentration on the TiO2 surface.