Here we examine the application of the COSMO-SAC model to phase equilibrium calculations of solutions containing ionic liquids (ILs) As there is uncertainty about the degree of ionization in these liquids, we consider three methods treating the IL as a neutral molecule, as a separated anion and cation, and essentially as an ion pair In the first case, the needed quantum mechanics calculation is done for the neutral molecule referred to as COSMO-SAC(molecule), and in the other two cases, the calculations are performed for the separated ions in which the IL is treated as two separate components, COSMO-SAC(ions) or COSMO-SAC(CA), in which the separate cosmo files are combined and the IL is treated as a single-component neutral ion pair There is generally good agreement between the predicted results and the experimental data using COSMO-SAC(CA) and COSMO-SAC(ions), while for COSMO-SAC(molecule) the results are less satisfactory and the calculations arc more difficult because of the problems with geometry optimization and the density functional calculations for the large Intact IL molecules The results show that the COSMO-SAC model can give reasonably good predictions for the vapor-liquid equilibria, liquid-liquid equilibria, and infinite dilution activity coefficients for solutions containing ionic liquids