In order to enhance the separation ability of ionic liquid (IL) in acetone + methanol azeotropic mixture, the isobaric vapor-liquid equilibrium (VLE) data for the ternary systems of acetone + methanol IL (1-propyl-3-methylimidazolium bromide [PMIM]Br, 1-ally1-3-methylimidazolium bromide [AMIM]Br or 1allyl-3-methylimidazolium chloride [AMIM]Cl) were measured at 101.3 kPa. The influences of different alkyl side chains or anions on the separation efficiency of IL were studied. The azeotropic point of the binary system is pulled up with the increase of IL content. The azeotropy is completely eliminated when IL content reaches a certain value. The separation efficiency of the three ILs follows the order: [AMIM] Cl > [AMIM]Br > [PMIM]Br. The experimental VLE data are well correlated by the nonrandom two-liquid (NRTL) model. (C) 2017 Elsevier B.V. All rights reserved.