Perfluorooctane sulfonate (PFOS) recently has been listed as a persistent organic pollutant due to its worldwide occurrence and environmental persistence. The methods for PFOS decomposition are limited due to its inertness. Here we report that PFOS in an anoxic alkaline solution (pH >= 8) efficiently decomposes and defluorinates under irradiation of 185 nm vacuum ultraviolet (VUV) with a low-pressure mercury lamp (23W, 1.89 mW/cm(2)). The branched isomers in the technical PFOS product decomposed very fast with the first-order reaction rate constant of 0.0806 min(-1) (t(1/2) = 8.6 min) at pH 12.5, while the linear PFOS decomposed slower with the rate constant of 0.0175 h(-1). The rate constants are nearly 967 times and 4 times higher than that at pH 6.8 (without pH adjustment), respectively. Hydrated electrons (e(aq)(-)) were confirmed as the active species for the PFOS decomposition. Few fluorinated intermediates were detected either in solution or in gas phase by ESI-MS and ATD-GC/MS, and high yield of fluoride (16.7 mol F-/mol PFOS) was achieved, which indicates that PFOS can be completely defluorinated by this method. This study represents the first attempt to evaluate the effect of VUV light on photochemical decomposition of PFOS in an anoxic alkaline solution, as well as finding that PFOS isomers have different reactivity to hydrated electrons, which offers new insights on designing the substitutes for the existing perfluorinated chemicals. (C) 2015 Elsevier B.V. All rights reserved.