The TiO2 photoassisted degradation of pesticide pyridaben has been examined in acetonitrile/water suspensions under UV light irradiation mainly at wavelength longer than 360 nm. To distinguish from the accompanying photolytic phenomenon, the kinetics of direct photolytic and photocatalytic degradation of pyridaben in TiO2 suspensions were studied by two different UV wavelength irradiations (lambda greater than or equal to 300 nm and lambda greater than or equal to 360 nm). The results manifested that the photolysis or photocatalysis of pyridaben followed pseudo-first-order kinetics, the direct photolysis was dominant at lambda greater than or equal to 300 nm, and the higher photocatalytic efficiency was obtained at lambda greater than or equal to 360 nm. The photocatalytic degradation kinetics were studied under different conditions such as water content, pH, catalyst concentration as well as radiant flux, and the degradation rates were found to be strongly influenced by these parameters. A qualitative study of the degradation products generated during the process was performed by GC-MS. Up to fifteen compounds were detected as degradation intermediates, many of which were identical with those detected previously at lambda greater than or equal to 300 run. We also analyzed the evolution of the degradation products semiquantitatively by plotting areas of the corresponding GC peaks as functions of irradiation time. Moreover, a complementary study using molecular model calculation was performed to forecast pyridaben's adsorption point on TiO2 particles and its weak position of molecular cleavage. On the basis of the analytical and kinetic results, a degradation mechanism was proposed. This work is significant to understand the photochemistry of pesticide pyridaben in the environmental treatment. (C) 2004 Elsevier B.V. All rights reserved.