The solubilities and selectivities for CO2, N-2 and CH4 in ionic liquid were predicted using a COSMO based activity coefficient model, COSMO-SAC method. The 1-alkyl-3-methylimidazolium cations were focused in this work. The anion species include tetrafluoroborate [BEd, hexafluorophosphate [PF6], triflate dicyanamide [dca] and bis(trifluoromethane)-sulfonimide [Tf2N]. The predicted results of the solubilities of CO2 in the ionic liquids by COSMO-SAC method are in agreement with the experimental data within the averaged deviation of 0.0017 in mole fraction. The predicted results of selectivities for CO2/N-2 and CO2/CH4 represent the effects of anion species qualitatively. Permeability through supported liquid membrane can be presented by solubility and diffusion coefficients in the liquid. The permeabilities of CO2 through the ionic liquid membranes were also predicted by a solution-diffusion model with COSMO-SAC method. The predicted results of the CO2 permeabilities through the ionic liquids represent the experimental data within the order of the permeabilities. (C) 2010 Elsevier B.V. All rights reserved.