Isobaric vapour-liquid equilibrium (VLE) for the (acetonitrile + water) system containing 1-hexyl-3-methylimidazolium chloride ([HMIM][Cl]), 1-butyl-3-methylimidazolium chloride ([BMIM][Cl]), 1-allyl-3-methylimidazolium chloride ([AMIM][Cl]) and 1-butyl-3-methylimidazolium acetate ([BMIM] [OAC]) are measured at atmospheric pressure (101.30 kPa). The experimental results show that the addition of ionic liquids (ILs) can improve the separation of acetonitrile and water and eliminate the azeotrope when the content of ILs reaches a certain value. The separation ability of ILs is [BMIM] [OAC] > [AMIM][Cl] > [BMIM][Cl] approximate to [HMIM][Cl]. A crossover effect of salting-in and -out of acetonitrile in different concentration ranges is observed. The experimental VLE data are satisfactorily correlated with the non-random two liquid (NRTL) model. Additionally, COSMO-SAC is employed to predict the phase behaviours of the investigated systems and good trends are obtained. (C) 2019 Elsevier Ltd.