The photocatalytic degradation of trichloroethylene and tetrachloroethylene in the gas phase, when porous TiO 2 pellets were used, produced such undesirable chlorinated byproducts as chloroform (CHCl 3 ) and carbon tetrachloride (CCl 4 ). The formation of byproducts was reduced by increasing oxygen mole fractions in the feed gas stream and/or using the TiO 2 pellets fired at a lower temperature. Theoretical calculations indicate that the Cl-radical-initiated reaction produces these byproducts. On the basis of the experimental and the theoretical results, a method to prevent the formation of the chlorinated byproducts is discussed.