The photochemical decomposition of perfluorodecanoic acid (PFDeA) in water in the presence of persulfate ion (S2O82-) and sulfur ion (S2-) was investigated under vacuum ultraviolet (VUV) light irradiation. PFDeA was decomposed under VUV light irradiation. With the addition of S2O82- or S2-, the photodecomposition and defluorination of PFDeA were enhanced significantly. Sulfate radical anion (SO4 center dot-) generated from photolysis of S2O82- initiated PFDeA oxidation. While the S2- ion, acting as a (OH)-O-center dot scavenger, enhanced the role of reduction pathway induced by aqueous electrons (e -). The shorter-chain perfluorocarboxylic acids (PFCAs), formed in a stepwise manner from longer-chain PFCAs, were identified as products by HPLC/MS. (C) 2010 Elsevier B.V. All rights reserved.