In this study, four photochemical reaction parameters, including the reaction atmosphere, extra inorganic oxidant, inorganic anion and organic matter, were investigated in an aqueous environment to determine their effects on perfluorooctanoic acid (PFOA) photodecomposition. The photochemical defluorination of PFOA was conducted by vacuum ultraviolet light (VUV) using ferric ions as the catalyst. Both direct photolysis and photocatalysis were involved in the reaction. PFOA was completely decomposed by VUV irradiation with and without ferric ions within 72 or 144 h, respectively. There was no significant evidence indicating that the type of atmosphere (oxygen, nitrogen, or air) had any effect on the decomposition results. However, (OH)-O-center dot radicals generated during the photolysis process profoundly affected the decomposition of PFOA. The introduction of persulfate inhibited the mineralization of PFOA in the VUV/Fe3+ system. Inorganic anions hindered the defluorination of PFOA in the following order: HCO3- > SO42- > NO3- similar to Cl- > ClO4-. The adverse effects of organic matter, including volatile organic compounds and dissolved natural organic compounds, were also examined. This study is beneficial to the understanding of the photodecomposition of other similar perfluorinated compounds (PFCs). (C) 2016 Elsevier B.V. All rights reserved.