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This study investigated the performance of UVC, UVC/ZnO and vacuum UV (VUV) processes for the degradation of diazinon as a model organophosphate pesticide. The highest diazinon degradation was obtained at a solution pH of 5 for UVC, 7.5 for UVC/ZnO, and 9 for VUV. At optimum pH and a reaction time of 30 min, the UVC process degraded 57.8% of the diazinon (10 mg/L) and the UVC/ZnO process degraded 93.3%. By comparison, the VUV process completely degraded 10 mg/L diazinon in a very short reaction time of 90 s. VUV produced a significantly greater degradation rate for diazinon than UVC and UVC/ZnO. Under similar operating conditions, the first-order degradation rate of 5 mg/L diazinon for VUV was 119 times greater than for UVC and 18 times greater than for UVC/ZnO. In continuous mode, the VUV process completely degraded 1 mg/L diazinon in natural water at a hydraulic retention time (HRT) of 2.2 min; complete mineralization was obtained at a HRT of 4.7 min. It was found that the VUV process is a very efficient and viable process for complete mineralization of organophosphate pesticides in water sources and is an appropriate technology for real scale applications. [ABSTRACT FROM AUTHOR]