Photocatalytic oxidation of phenol in water was carried out with nanoparticles of silica-titania mixtures, which were synthesized under different temperatures and silica-to-titania ratios. The crystal size of TiO2 (in anatase phase) was determined to be in the nanometer range and it increased with increasing autoclaving temperature. Furthermore, there was no obvious relationship between the size and the SiO2/TiO2 ratio at the same preparation temperature. A specific reaction rate constant (k(s)) was used for comparison of photocatalytic activity of different samples. It was found that k(s) decreases with increasing anatase size and TiO2 concentration. A kinetic model was developed to describe the effect of the crystal size and titania concentration on the reactivity of the SiO2-TiO2 samples.