The photooxidation of trichloroethylene (TCE) on TiO2 has been investigated using infrared spectroscopy for kinetic studies of the production of intermediate species and for investigation of the reaction mechanism. Trichloroethylene is oxidized by chemisorbed molecular O-2 when TiO2 band gap radiation (hv > 3.1 +/- 0.1 eV) is incident. An intermediate species, dichloroacetyl chloride, HCl2CCOCl, was identified. At 300 K, Cl2CO, CO, CO2, HCl, and H2O are final photooxidation products. At 473 K, Cl2CO undergoes a thermal side reaction on TiO2 to produce Cl2C=CCl2. Studies in which oxygen-labeled H2O was present indicated no incorporation of the oxygen label into any of the intermediates, showing that the OH. driven oxidation mechanism proposed by others is not operative. At 150 K, TCE blocks TiO2 sites, significantly retarding O-2 adsorption and slowing the photooxidation reaction.