The photodegradation of organophosphorus pesticides has an important influence on their fate and bioavailability in the water environment. In this study, the kinetics and mechanisms of diazinon photodegradation by Fe (III)-oxalate complex have been determined. Special attention was given to the pathway by which phosphate is released following diazinon photodegradation, as assessed by HPLC-ESI-Q/TOF-MS coupled with oxygen isotope. The results showed that diazinon was stable under dark treatment. However, the degradation of diazinon was observed in the UV-only, UV-Fe(III), and UV-Fe(III)-oxalate treatments. The degradation rate constant is the largest in the UV-Fe(III)-oxalate treatment and clearly influenced by the pH and Fe(III) / oxalate ratio. The hydroxyl radical (center dot OH) was the main reactive oxygen species (ROS) in the UV-Fe(III)-oxalate complex treatment and the steady-state concentration of center dot OH was 5.75 x 10(-14) M. The products analysis revealed that phosphate could be released during the photodegradation of diazinon; the intermediate products were diazonon, 2-hydroxydiazonon, hydroxydiazonon, hydrogen phosphorothioate, O,O-diethyl thiophosphate (DEW), diethyl phosphate (DEP) and pyrimidinol (IMP). Compound stable oxygen isotope analysis coupled to Q-TOF/MS revealed that the degradation of diazinon initiated by the P-O bond cleavage.