Thermal treatment in air has a marked influence on the adsorption of oxygen and the photocatalytic activities of TiO2 (P25) for the oxidation of acetone in air. The photoactivity of TiO2 increases with the increase of thermal treatment temperature until 400 degrees C, above which more rutile is formed. On the basis of the photocatalytic performance of TiO2 thermally treated in a vacuum and the results of polycrystalline X-ray diffraction analysis, BET surface area: adsorption oxygen measurements and ESR spectra, the increase in photoactivity can be attributed to a catalyst surface with more adsorbed oxygen. This would generate more superoxide anion radicals under UV irradiation, resulting in a better separation of photoexcited electrons and holes.