The photoassisted catalytic degradation of trichloroethylene (TCE) in the gas phase was studied using a packed bed reactor with TiO2 pellets prepared by sol-gel techniques. A primary product of the reaction was monochloroacetic acid at 23 degrees C while TCE was completely mineralized to CO2 and HCl at 64 degrees C. Ab initio molecular orbital calculations served to elucidate the mechanism of the TCE degradation. According to the frontier molecular orbitals of TCE, on the catalyst surface the OH radical attacks preferentially at the CCl2 side of TCE, path A. The formation of the radical intermediate CHClCCl2(OH) was estimated to be exothermic by -41.0 kcal mol(-1). The large stabilization energy is in agreement with the experimental results that TCE is degraded very rapidly and the reaction was not dependent on the temperature of the reaction. A Cl radical elimination from the intermediate produces 1,2-dichloroethenol, which changes its form to monochloroacetyl chloride. This is the so-called keto-enol tautomerism with the large activation energy of 53.3 kcal mol(-1) The water assists by lowering the activation barrier of the tautomerism by 21.5 kcal mol(-1). Hydrolysis of monochloroacetyl chloride or its reaction with a OH radical produces monochloroacetic acid, the primary product pf our catalytic system. The addition of OH radical to the CHCl side favorably occurs in the gas phase, path B, and forms a radical intermediate CHCl(OH)CCl2. Then, it releases a Cl radical to form 2,2-dichloroethenol followed by the tautomerism to produce dichloroacetaldehyde. The activation energy of the reaction turned out to be as large as 64.2 kcal mol(-1). Such a high activation barrier is responsible for the previous observation that only 2,2-dichloroethenol was detected in the homogeneous gas phase degradation of TCE.