The atmospheric oxidation of trichloroethylene has previously been studied experimentally. Phosgene is thought to be the dominant product, although the mechanism of production is not well understood. Additionally, studies omitting a chlorine scavenger show the production of dichloroacetyl chloride. This influence of the chlorine atom on the trichloroethylene oxidation is not well understood. Using ab initio methods, this study presents a comprehensive computational study of both the hydroxyl radical and chlorine atom initiated atmospheric oxidation mechanisms of trichloroethylene (C2HCl3). Potential energy surfaces, including activation energies and enthalpies, are determined. The results from this study, in connection with experimental work, confirm the influence of the Cl-initiated oxidation in determining the product profile of the trichloroethylene oxidation. These products include dichloroacetyl chloride [Cl2CHC(O)Cl], formyl chloride [CH(O)Cl], phosgene [C(O)Cl-2], and regeneration of the chlorine atom.