A model reaction has been employed to investigate the mechanism for the photooxidation of chlorinated hydrocarbons on TiO2 semiconductor surfaces. Utilizing modern surface science techniques, the roles of surface defects (surface oxygen vacancies), adsorbed molecular oxygen, rind surface hydroxyl groups have been studied on TiO2(110) in connection with the photooxidation of methyl chloride. The coverage of each of the above species can be characterized and well controlled. The presence of surface defects and molecular oxygen adsorbed on the defect sites is essential for the photoreaction to initiate and to proceed. Surface hydroxyl groups do not oxidize methyl chloride under UV irradiation, However, adsorbed water molecules do participate in the photoreaction in the presence of adsorbed molecular oxygen. The photooxidation has an energy threshold of 3.1 eV, showing that the reaction is induced by TiO2 band-gap excitation. These results show that under our experimental conditions the substrate-mediated excitation of molecular oxygen plays the more important role in the photooxidation of methyl chloride on TiO2 surfaces.