In this paper, the photocatalytic degradation of trichlorfon, an organophosphorous pesticide, was studied by using TiO2 as a photocatalyst. The effects of various parameters, such as the amount of the photocatalyst, illumination time. reaction temperature, electron acceptors, metal ions, anions, and initial pH value on the photocatalytic degradation of trichlorfon were investigated. The best conditions for the photocatalytic degradation of trichlorfon were obtained. The results show that the optimum amount of the photocatalyst used is 8.0 gL(-1). The photodegradation efficiency of trichlorfon increases with the increase of the illumination time or reaction temperature. The photodegradation efficiency of trichlorfon is increased rapidly by adding a small amount of H2O2, K2S2O8, KBrO3, Fe3+ and Cu2+, however, with the addition of Na+, K+, Mg2+, Ca2+, Zn2+, Co2+ and Ni2+, or with the addition of trace amount of SO42-, Cl-, Br-, there are no obvious effects on the photocatalytic degradation reactions. Alkaline mediums are favorable for the photocatalytic degradation of trichlorfon. The possible roles of the additives on the reactions and the possible mechanisms of effect were also discussed. (C) 2009 Published by Elsevier B.V.