Decomposition of perfluorooctanoic acid (PFOA) was investigated in UV-Fenton systems. It was found that PFOA could be efficiently degraded in the UV-Fenton system, producing fluoride ions and short-chain perfluorinated carboxylic acids. The degradation of PFOA could be divided into two stages: Within reaction time of 1 h, the added PFOA was rapidly degraded, resulting in a PFOA removal of 87.9% and a defluorination efficiency of 35.8%; Beyond 1 h, the PFOA removal was slowly increased to more than 95% at reaction time of 5 h, along with the defluorination efficiency of 53.2%. This corresponded to a two-stage mechanism for the PFOA degradation process in the UV-Fenton system. The fast defluorination and degradation of PFOA at the first stage was attributed to the generation of strong oxidizing (OH)-O-center dot radicals, which accelerated PFOA decarboxylation. At the second stage, H2O2 was completely consumed, and the defluorination was mainly due to the interactions between PFOA and Fe3+ ions. (C) 2012 Elsevier B.V. All rights reserved.