A simple model was developed to predict the solubility of carbon dioxide (CO2) in ionic liquids under normal processing and conditioning pressures for natural gas fluids. COSMO-RS was used to predict the unsymmetrical activity coefficients for CO2 in ILs, and the Peng-Robinson equation of state was used to calculate the CO2 fugacity coefficient. Using these two quantities, a correlation was developed which can be used to extrapolate solubilities for ILs where experimental data is not available. This study shows that the molecular mass of the ILs is an effective parameter to correlate the Henry's constant of CO2. (C) 2011 Elsevier B.V. All rights reserved.