The direct photolysis and the photocatalytic degradations of chlorfenapyr in TiO2 suspensions with and without the use of hydrogen peroxide were studied using two different monochromatic UV irradiations (300 and 350 run). Both the direct photolysis and photocatalytic degradations of chlorfenapyr follow pseudo-first-order degradation kinetics (C-t = C(0)e(-kt)). Photolysis reactions were slow, and the corresponding photocatalysis rates were increased by about 2.5 and 3 times in the presence of TiO2 at 300 and 350 nm of UV, respectively. Photocatalytic rates were increased with the pH at neutral to alkaline ranges because of the increase of hydroxide ions. However, the reaction was gradually retarded at the neutral medium. The half times of degradation (t(1/2)) were 13.7, 16.2, 16.2, 10.7 and 8.7 min, respectively, at pH 4, 6, 7, 9 and 11. The effect of temperature is helpful to the photocatalytic degradation of chlorfenapyr. The photocatalytic degradation rates were found to increase with TiO2 dosages, and the reaction was retarded at higher TiO2 dosages. The fastest degradation occurred when 1200 mg L-1 TiO2 was used, the rate increased slowly with increasing catalyst concentration up to 800 mg L-1 and more rapidly when the concentration was increased from 800 to 1200 mg L-1, and decreased at 1600 mg L-1. A much faster degradation rate at lower chlorfenapyr concentrations was achieved. The results of H2O2-assisted photocatalysis experiments showed that a low H2O2 dosage in photocatalysis would enhance the degradation rate of chlorfenapyr, however, an overdose of H2O2 will retard the rate because of the expenditure of hydroxyl radicals. (c) 2005 Elsevier B.V. All rights reserved.