The presence of dissolved organic matter (DOM) in aquatic system has an important influence on the phototransformation of organic contaminants through the production of reactive substances, such as hydroxyl radicals ((OH)-O-center dot), singlet oxygen (O-1(2)), and DOM triplet states ((DOM)-D-3*) under solar irradiation. Addition of 5 mg/L of Humic acid sodium (HA), Suwannee River NOM (SRNOM) and Nordic Reservoir NOM (NRNOM) all accelerated the photodegradation of 2,4',5-trichlorobiphenyl (PCB 31) significantly, with a pseudo-first-order rate constant of 0.0933, 0.0651 and 0.0486 in the initial 12 h, respectively. HA and SRNOM, the allochthonous DOM, showed higher reactivity in the photolysis of PCB 31. The maximum photodegradation rate was observed in 5 mg/L of DOM solution. The roles of the reactive substances were studied by the inhibitory experiments, which suggested that (OH)-O-center dot and intra-DOM O-1(2) were more important for the photolysis of PCB 31 than other reactive substances, accounting for 35.1% and 47.1% of the degradation, respectively. The main degradation products of PCB 31 detected by GC-MS were 4-chlorobenzoic acid, 2,5-dichlorobenzoic acid, hydroxy-2,5-dichloroben:zoic acid, 4-hydroxy-2',5'-dichlorobiphenyls and hydroxy-trichlorobiphenyls. The degradation pathways were accordingly proposed. Photosensitized degradation by DOM, especially the intra-DOM reactions, may be a very important mechanism for the transformation of PCBs and other hydrophobic organic contaminants in the environment. (C) 2011 Elsevier B.V. All rights reserved.