The transformation of organic phosphorus plays an important role in the phosphorus cycle in the natural environment. In this study, a series of laboratory-based experiments were conducted to address the influence of nitrate (NO3-) photochemical activity on the transformation of methyl parathion under solar irradiation. It is demonstrated that the photodecomposition of methyl parathion by NO3- obeys pseudo-first-order reaction kinetics, with the photodecomposition rate increasing with NO3- concentration. The photodecomposition rates of methyl parathion by NO3- were strongly influenced by pH, concentration of humic acid and Fe 31. Higher concentrations of Fe 31 and lower concentrations of humic acid accelerated the photodegradation induced by NO3-. Moreover, compared to alkaline media the acidic media can enhance the rate of methyl parathion degradation. Formation of the hydroxyl radical (center dot OH) was identified through the measurement of photoluminescence spectra (PL) using terephthalic acid as the trapping molecule. The primary transformation pathways of methyl parathion in the presence of NO3- under solar irradiation were proposed through the qualitative and quantitative analysis of byproducts. Intermediates, such as paraoxon, 4-nitrophenol and orthophosphate, were identified, and a mineralization pathway from methyl parathion to orthophosphate was proposed. (C) 2016 American Institute of Chemical Engineers