The photooxidation of organophosphorus pesticides (OPP) such as methamidophos, phorate, malathion, diazion and EPN, was investigated in UV-TiO 2 , UV-H 2 O 2 and UV-TiO 2 -H 2 O 2 systems. Apparent first-order rate constants were used to describe the photodecomposition behaviors of OPP. Batch results demonstrate that the degradation efficiencies of OPP increased in the order of phorate > methamidophos > malathion diazion > EPN. The photooxidation of methamidophos and phorate is primarily attributed to direct photolysis. Whereas the presence of TiO 2 promotes the degradation efficiencies of diazion, malathion and EPN. Hydrogen peroxide was found to be an intermediate in UV-TiO 2 system during the photocatalytical reaction process. Longer irradiation decreased H 2 O 2 concentration due to the high electronhole recombination rate on the surface of TiO 2 . Adding H 2 O 2 is more effective on the photocatalytic oxidation of OPP than TiO 2 . Threefold to tenfold of the quantum efficiency of the UV-H 2 O 2 system was obtained for the UV-TiO 2 system. Moreover, the quantum efficiencies for degradation of OPP decreased when light intensity is increased from 100 to 450 W, although increasing light intensity can increase the apparent first-order rate constants of OPP.