The adsorption and photoreactions of CH2Cl2 on powdered TiO2 have been investigated by Fourier transform infrared spectroscopy. CH2Cl2 is adsorbed molecularly or dissociatively to form chloromethoxy (CClH2O(a)) at 35 degreesC. CClH2O(a) decomposes into CH3O(a) and HCOO(a) in a vacuum at temperatures higher than similar to100 degreesC. As TiO2 in contact with gaseous CH2Cl2 is heated in a closed cell, HCl from Cl-(a) and OH(a) recombination, CH2O from CH3O(a) decomposition, and CO and CO2 from HCOO(a) decomposition are detected. In the CH2Cl2(a) photodecomposition in the absence of O-2, CO(a), CO3(a), and HCOO(a) are generated. The TiO2-mediated CH2Cl2(a) photodecomposition is likely initiated by surface-active oxygen species instead of direct hole transfer. In the presence of O-2, CH2Cl2(a) photodecomposition is accelerated. In addition to CO(a), CO3(a), and HCOO(a), H2O(a) and CO2(a) are generated as well. In the case of CO(a) formation, O-2 is also involved in addition to TiO2 lattice oxygen. The O-2 participation may be via oxygen anion species.