Photocatalytic degradation of pyridaben under UV irradiation has been investigated in acetonitrile/water solution containing TiO2 particles. The primary degradation of the pollutant followed the Langmuir-Hinshelwood model with kinetic constant k, 4.3 x 10(-5) mol/l min and equilibrium adsorption constant K, 3.1 x 10(3) l/mol. Eight kinds of degradation products (DPs) were identified by GC-MS in the process of reaction and some of them were further confirmed by matching with authentic standards and synthesized compounds. Structure identification of DPs allowed us to propose the main degradation route. The key step was suggested to be the cleavage of C-S bond between phenyl ring and heterocyclic group. Meanwhile, frontier electron densities (FED) of pyridaben were theoretically studied on the basis of PM3 calculations for a better understanding of the reaction mechanism. The result indicated that the highest FED position was on the S atom of pyridaben molecule, where should be the starting point of the photocatalytic reaction, which is consistent with the experimental result. (C) 2004 Elsevier B.V. All rights reserved.