The ultraviolet (UV) photolysis of atrazine in aqueous solution was investigated at wavelength of 254 nm in this study. This paper was mainly focused on the identification of atrazine degradation intermediates by HPLC-MS/MS and its degradation mechanisms. The photodegradation products included the following seven classes: dechloro-hydroxylated products, chloro-dealkylated products, dechloro-dealkylated products, alkylic-oxidated products, delamination-hydroxylated products, olefinic products, and dechloro-hydrogenated products which were never reported in direct photolytic process, 4-isopropylamino-6-ethylamino-s-triazine (IEST), 4,6-dihydroxy-s-triazine (OOST). The main degradation products were 2-hydroxy-4-acetamido-6-ethylamino-s-triazine (OlET), 2-chloro-4-isopropyl-amino-6-methylamino-s-triazine (CIMT), 2-chloro-4,6-divinylamino-s-triazine (CVVT). 2-chloro-4-ethylamino-6-amino-s-triazine(CEAT). 2-methoxy-4-isopropyl-amino-6-methylamino-s-triazine(OIMT), 2-hydroxy-4-acetamindo-6-ethylamino-s-triazine (ODET), etc. Finally, the possible degradation mechanism was also proposed here. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.