A model for the kinetics of photocatalytic processes on spherical TiO2 particles is developed. The quantum yield for the photooxidation of organic molecules by, oxygen in aqueous solution is derived for two cases. In one case the yield is limited by surface recombination, in the other case by a reverse reaction of electrons with the first oxidation product by holes. It is shown that the quantum yield increases with decreasing illumination intensity and size of the particle.