Fluoxetine (FLX) was shown to be photoreactive in sunlit surface waters. FLX degraded in deionized water when exposed to simulated sunlight with a half-life of 55.2+/-3.6 h(-1). Photodegradation products were identified using high performance liquid chromatography-UV (HPLC-UV) and liquid chromatography tandem mass spectrometry (LC-MS-MS) using electrospray (ES) ionization. Defluorination of the trifluoromethyl group in FLX and in fluometuron and flutalanil,two other compounds containing this functional group, is suggested to be a common direct photolysis pathway for trifluoromethylated compounds. Products resulting from O-dealkylation of FLX were also observed. The rate of degradation was faster in synthetic field water where .OH was the likely dominant oxidant in the system. The bimolecular rate constant for the reaction between FLX and .OH was measured as (8.4+/-0.5) x 10(9) and (9.6 +/-0.8) x 10(9) M(-1) s(-1) using two different methods of competition kinetics. Indirect photodegradation reactions could lead to the production of hydroxylated and O-dealkylated compounds. Although direct photolysis could potentially limitthe persistence of FLX in surface waters, its degradation by indirect photolysis would proceed faster. Thus, this latter process could be important in the elimination of FLX in surface waters.