The photochemical decomposition of perfluorooctanoic acid (PFOA) in aqueous periodate (IO4-) was investigated under two types allow-pressure mercury lamps: one emits at 254 nm light (UV light) and the other emits both 254 nm and 185 nm light (VUV light). PFOA decomposed efficiently under VUV light irradiation while it decomposed poorly under UV light irradiation. The additional IO4- significantly increased the rate of decomposition and defluorination of PFOA irradiated with UV light whereas it decreased both processes under VUV irradiation. Reactive radical (IO3 center dot) generated by photolysis of IO4- initiated the oxidation of PFOA in UV process. Aquated electrons (e(aq)(-)), generated from water homolysis, scavenged IO4-resulting in decrease of reactive radical species production and PFOA decomposition. The shorter-chain perfluorocarboxylic acids (PFCAs) formed in a stepwise manner from long-chain PFCAs. (C) 2010 Elsevier B.V. All rights reserved.