Perfluorooctanoic acid (PFOA) could be efficiently degraded by the combined process of photocatalysis and ozonation, its degradation intermediates were fluoride ions and short-chain perfluorocarboxylic acids (PFCAs) including C6F13COOH, C5F11COOH, C4F9COOH, C3F7COOH and C2F5COOH. After 4 h reaction time, its degradation efficiency was 99.1% and defluorination ratio only reached to 44.3%. The degradation mechanism of PFOA by photocatalytic ozonation was proposed and found to be similar to Photo-Kolbe reaction: PFOA anion was degraded to produce PFOA radical by a direct electron transfer from its carboxyl group to photo-generated hole (h(+)). Then PFOA radical was decarboxylated to produce perfluoroheptyl radical, and then be oxidized to form C2F15OH by hydroxyl radical ((OH)-O-center dot) which was produced from ozone trapping photo-generated electron. Because of its thermal instability, C7F15OH would be converted to C6F13COF, which was further hydrolyzed to generate C6F13COO-. Therefore the combined process of photocatalysis and ozonation was efficient for PFOA degradation. (C) 2016 Elsevier B.V. All rights reserved.