The photooxidation of CH3Cl on TiO2 has been investigated using two approaches. In the first, TiO2(110) (rutile) containing oxygen anion vacancy defect sites has been studied using temperature-programmed desorption to follow the production of photooxidation products. In the second approach, powdered TiO2 (mainly anatase) has been studied using transmission infrared spectroscopy.; On TiO2(110) the observed products are H2CO, CO, H2O, and HCl. On powdered TiO2, the products are CO, CO2, H2O (as surface OH groups), HCl, and CH2Cl2. The dissimilarities are postulated to be due to two factors : (1) different surface site distributions for the two types of TiO2 surface; (2) higher pressure of reactants in the powdered TiO2 investigation. Both sets of experiments were done in the absence of water and demonstrate that water is unnecessary for the photooxidation reaction. However, O-2 is crucial for the photooxidation of CH3Cl. The formation of CH2Cl2 as one of the photooxidation products over powdered TiO2 suggests that a free-radical-mediated mechanism exists in this case.