We propose a new predictive approach for the calculation of properties and phase behavior of mixtures containing ionic liquids (ILs). In this method the Pitzer-Debye-Huckel (PDH) model for the long-range electrostatic interactions is combined with the COSMO-SAC model for the short-range interactions between contacting molecules. While the COSMO-SAC model alone has been shown to be accurate for vapor-liquid equilibrium (VLE) and infinite dilution activity coefficient (IDAC) of solvents in ILs, it is inaccurate for dilute IL solution properties such as the osmotic coefficient (OC) and mean ionic activity coefficient (MIAC), where long-range interactions are important. The PDH model alone is accurate for dilute IL solutions; however, it is not applicable at high IL concentrations. Here we propose a novel method for attenuating the long-range effects from the PDH model at high IL concentrations. The resultant model is found to be accurate over the whole concentration ranges of the IL solutions. We have examined this method for three versions of the COSMO-SAC model, denoted as COSMO-SAC(2007) + PDH(alpha), COSMO-SAC(2010) + PDH(alpha), and COSMO-SAC(ion,alpha), for mixtures involving 73 ILs formed from the combination of 20 cations and 15 anions. The absolute relative deviations (ARD) from the three models for IDAC (3552 points) are 107.89%, 165.37%, and 116.94%, respectively. The errors for VLE (1626 points) are 23.98%, 28.88%, and 22.02%, respectively. The prediction accuracy for OC (1348 points) are 40.86%, 48.10% and 24.09%, and the predicted MIAC (484 points) of ionic liquid showed a similar accuracy of about 48%. The proposed COSMO-SAC models provide prediction of properties of IL fluids with consistent accuracy over the full range of concentrations. (C) 2013 Elsevier B.V. All rights reserved.