The gas-phase photocatalytic degradation of cis-1,2-dichloroethylene (cis-DCE) using TiO2 under near-ultraviolet (UV)-illumination was studied by means of Fourier transform infrared (FT-IR) spectroscopy and GC-MS. The detected and possibly determined products were the following: CO as a main intermediate, CO2 and HCl as the major final products, dichloroacetyl chloride, 1,1,2,2-tetrachloroethane, dichloromethane, oxalyl chloride, and phosgene as the by-products. These by-products except phosgene were finally oxidized to CO2 and HCl. In this study we proposed the reaction schemes to give a rational explanation for formation of the above products. In the degradation of cis-DCE unlike trichloro- (TCE) and tetrachloroethylene (PCE), the Cl-elimination hardly occurred and therefore the reactions with Cl atoms, involving a chain-propagation step, seem not to be significant steps. In the degradation of cis-DCE in the presence of PCE, the degradation rate and each yield of by-products except oxalyl chloride increased. These findings accept the explanation that an increase of Cl atoms by PCE addition facilitated the long-chain processes and enhanced the reactions with Cl atoms. The yield of CO was found to rise with increasing of the initial concentration of cis-DCE. The high CO yield inhibited the complete oxidation of cis-DCE. The oxidation Of CO to CO2 was enhanced in the presence of PCE, and therefore the addition of PCE also improved the conversion of especially high concentration of cis-DCE into CO2. (C) 2004 Elsevier B.V. All rights reserved.