In this study, the performance of the combined UV/O-3 process for the degradation of microcystin-LR (MC-LR) in water was investigated, and the possible degradation pathway of the toxin was elucidated according to reaction intermediates. Results demonstrated that the UV/O-3 process was a more effective method for the removal and mineralization of MC-LR in water, compared with UV- and O-3-alone processes. Increasing the solution pH (5-10) and dissolved organic carbon concentration (0-5.3 mg/L) could inhibit the degradation of the toxin. Nine reaction intermediates of MC-LR were detected by LC/MS, and four initial reaction sites of the toxin were observed: the conjugated diene of Adda, the double bonds of Mdha and two free acid groups of MeAsp and Glu. The degradation pathways of the toxin during UV/O-3 process involved isomerization, hydroxylation and oxidative cleavage of the Adda side chain, oxidation of Mdha and decarboxylation of MeAsp and Glu. The pathway of the oxidation and cleavage of Adda and Mdha could result from both HO center dot and O-3 oxidation, but the isomerization of Adda moiety and the decarboxylation of MeAsp and Glu mainly resulted from UV radiation. The yield trends of the intermediates indicated that the oxidation of Adda moiety was the dominant degradation pathway and that the combined UV/O-3 process had a higher potential for the simultaneous degradation of MC-LR and its intermediates than the individual processes. (C) 2015 Elsevier B.V. All rights reserved.