Three kinds of Bronsted acidic ionic liquids caprolactam benzenesulfonate ([CP]BSA), caprolactam hydrosulfate ([CP]HSO4) and caprolactam p-toluenesulfonate ([CP]pTSA) were synthesized. The structures of the ILs were confirmed by (HNMR)-H-1 and FT-IR. Regioselective mononitration of chlorobenzene has been investigated in HNO3-Ac(2)Osystem with three ionic liquids. The sequence of the nitrification activity is [CP]HSO4 > [CP]pTSA > [CP]BSA. With the caprolactam hydrosulfate as the catalyst the influences of reaction time, reaction temperature and the amount of the catalyst were studied in the nitration of chlorobenzene. The results showed that the optimum reaction conditions were as follows: when the mole ratio of chlorobenzene to ionic liquid was 10:1.5, the reaction temperature was 60 degrees C and the reaction time was 2.5 h, the yield of mononitro-chlorobenzene could reach 71.22%. And the mass ratio of para/ortho isomer was 7.74, which was much more than 2.0 of para/ortho isomer mass ratio obtained by nitric-sulfuric mixed acids as catalyst. Besides, the ionic liquid could be used repeatedly. Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.