For selection of ionic liquids (ILs) which can be potentially used as entrainers for separation of the azeotropic mixture of (water + ethanol) by extractive distillation, (vapor + liquid) equilibrium was measured for the ternary systems of (water + ethanol + an IL) using a previously proposed ebulliometer. The experimental measurement was performed at p = 100 kPa and in a way of continuous synthesis, in which analysis of liquid phase composition was avoided. While the mole fraction of ethanol calculated on IL-free basis, x'(2), was kept almost unchanged at 0.95, isobaric T,x,y data were measured at different IL mass fractions. Activity coefficients, as well as relative volatilities, of the volatile components were obtained from the experimental data without the need of a thermodynamic model of the liquid phase. There were eight ILs in our investigation: 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), 1-ethyl-3-methytimidazolium tetrafluoroborate ([emim][BF4]), 1-butyl-3-methylimidazolium dicyanamide ([bmim][N(CN)(2)]), 1-ethyl-3-methylimidazolium dicyanamide ([emim][N(CN)2]), 1-butyl-3-methylimidazolium chloride ([bmim][CI]), 1-ethyl-3-methylimidazolium chloride ([emim][CI]), 1-butyl-3-methylimidazolium acetate ([bmim][OAc]), and 1-ethyl-3-methylimidazolium acetate ([emim][OAc]). The effect of the ILs on the relative volatility of the volatile components was depicted separately by their effect on the activity coefficients. The results indicated that, among the eight Its studied, [emim][CI] has the largest effect on enhancement of the relative volatility. Another IL, [emim][OAc], has also significant effect. Considering the relatively low viscosity and melting point of [emim][OAc], this IL might be favorable candidate as entrainer for potential industrial application. (C) 2008 Elsevier Ltd. All rights reserved.