In this work, COSMO-RS model has been used in prediction of activity coefficients at infinite dilution of about 60 different organic solutes and water in 12 ionic liquids as well as ternary liquid–liquid equilibrium in 13 systems: (ionic liquid + methanol + heptane), (ionic liquid + thiophene + heptane) and (ionic liquid + benzothiophene + heptane). The model predictions are compared with experimental data (obtained in previously in our laboratory) in order to evaluate the accuracy of COSMO-RS model in phase equilibrium prediction of this kind of systems.