This study represents the first use of ultraviolet (VUV, lambda = 185 nm)-assisted ultrasound (US, f = 600 kHz) irradiation for the defluorination of aqueous perfluorooctanesulfonate (PFOS) under air atmosphere. The effects of reaction time, pH, temperature and initial PFOS concentration on the defluorination performance of VUV, US and VUV-assisted US systems were compared. The intermediates formed during PFOS defluorination in both the US and VUV-assisted US system were detected using LC/MS/MS. The results showed that introduction of VUV enhanced the ultrasonic defluorination efficiency of PFOS and the enhanced extents were depended on particular reaction condition. The defluorination efficiency of PFOS (10 mg/L) reached up to 88.47% after 240 min at 283 K under VUV-assisted US irradiation, which is 12.01% higher than that achieved under US irradiation alone. Compared with that achieved by US irradiation alone, the defluorination efficiency of PFOS (10 mg/L) increased by an average of 11.54% at pH levels of 5-7, 13.03% at 283-298 K and 11.65% at initial PFOS concentrations of 10-30 mg/L using the VUV-assisted US system. Low levels of short-chain perfluorinated carboxylic acids (PFCAs) were detected as the main intermediates in both the US and VUV-assisted US systems, and the concentrations of intermediates in VUV-assisted US system were lower than that in US alone system. The introduction of VUV radiation could provide more multiple approaches for defluorination of both the PFOS and its intermediates, thus improving the ultrasonic defluorination of PFOS under air atmosphere. (C) 2013 Elsevier B.V. All rights reserved.