TiO2 prepared by different methods was used to obtain anatase powders with particle sizes ranging from 5 to 165 rim. Powders were prepared using two methods: (i) a flame aerosol process in which nanoparticles of TiO2 were synthesized in a flame on oxidation (combustion) of an organotitanium precursor, and (ii) calcination of commercially available nanostructured TiO2 (Ishihara ST-01, 5 nm, as received). For comparison, Degussa P25 and Aldrich anatase TiO2 powders were also used as received. The TiO2 powders were characterized using XRD to determine crystal phase and crystal size, BET surface area analyses, uv-vis absorbtion spectroscopy to determine the band gap, and experiments in which the photoxidation rate of phenol in water was used as a measure of photoactivity. A model was developed based on the mechanistic steps in photocatalysis to elucidate the role of particle size on the apparent photoactivity of TiO2 for the photooxidation of organic substrates in water. The model was used to explain the trends in the experimental data for four different sets of photoactivity experiments with TiO2 powders. The results of this study elucidate a strong effect of particle size on photoactivity. The effects of particle size on the efficiency of light absorption and scattering and charge-carrier dynamics at particle sizes less than 25 nm dominate the apparent photoactivity of TiO2, and an optimum particle size of 25 to 40 nm exists within all sets of photocatalysis experiments conducted with TiO2 powders in this study. The optimum particle size is a result of competing effects of effective particle size on light absorption and scattering efficiency, charge-carrier dynamics, and surface area.