In this paper, the photocatalytic degradation of methamidophos, an organophosphorous pesticide, was investigated in aqueous solution by using TiO2 as a photocatalyst. The degradation was studied under different conditions such as the amount of the photocatalyst, illumination time, pH of the system, reaction temperature, initial concentration, electron acceptors, metal ions and presence of anions. The results showed that the photocatalytic degradation of methamidophos was strongly influenced by these parameters. The best conditions for the photocatalytic degradation of methamidophos were obtained. The optimum amount of the photocatalyst used is 12.0 g/L. The photodegradation efficiency of methamidophos increases with the increase of the illumination time. Alkaline media are favorable for the photocatalytic degradation of methamidophos. The degradation efficiency is enhanced by increasing reaction temperature, and the photodegradation efficiency decreases with the increase in the initial concentration of methamidophos. The photodegradation efficiency of methamidophos is accelerated by adding a small amount of H2O2, K2S2O8, KBrO3, Fe3+ or Cu2+. There are no obvious effects on the reactions with the addition of a small amount of Na+, K+, Mg2+, Ca2+, Zn2+, Co2+ and Ni2+ or adding trace amount of SO42-, Cl-, Br-. The possible roles of the additives on the reactions and the possible mechanisms of effect were also discussed. (C) 2008 Elsevier B.V. All rights reserved.