Fluorotrichloromethane (CFC 11) is, dehalogenated. through a reductive chain reaction upon illumination of aqueous, air-free suspensions of TiO2 particles in the presence of formate ions. The reduction takes place with large photonic efficiencies at pH greater than or equal to 5 even at high photon fluxes, producing mainly Cl- and dichlorofluoromethane (HCFC 21), while F- is only a minor byproduct. In the proposed mechanism (CO2-)-C-. and (CCl2F)-C-. radicals are the chain carriers, Cl- as well as HCFC 21 result from propagation steps, and cross-termination of the chain carriers forms F-., Simple steady-state assumptions, that regard propagations as the, dominant steps, yield kinetic equations consistent with the data of the initial fast Cl- formation step. The subsequent evolution of rates and postirradiation effects are consequences of the slow removal of electrons from the semiconductor.