The photocatalyzed degradation of the biocides chlorothalonil and dichlofluanid has been investigated in aqueous suspensions of titanium dioxide (TiO2) under simulated solar irradiation. The primary degradation of the micropollutants follows a pseudo-first-order kinetics following the Langmuir-Hinshelwood model. In our conditions, total disappearance of chlorothalonil and dichlofluanid was achieved in 90 and 20 min, respectively, whereas the mineralization of organic carbon to carbon dioxide after 240 min of irradiation was found to be 100% for chlorothalonil and 78% for dichlofluanid. The evolution of heteroatoms (Cl, N, S, F) followed by ion chromatography showed a mineralization into chloride, ammonium, nitrate, sulfate and fluoride anions, respectively. In addition, Microtox bioassay (Vibrio fischeri) was employed in evaluating the ecotoxicity of solutions treated by photocatalysis. Photocatalytic intermediates detected during the degradation of biocides were identified by GC-MS techniques. Based on this byproduct identification, a simple degradation scheme was proposed for both biocides including dechlorination, hydroxylation, dealkylation and decarboxylation reactions leading to the mineralization of the starting molecules. (C) 2003 Elsevier B.V. All rights reserved.