Ionic liquids (ILs) have achieved special and dedicated attention from the scientific community in recent years and a large number of studies involving different features of properties and applications of ILs have been presented. The complete understanding of the phase behaviour of lLs with water is an important issue yet there are few experimental data on their phase equilibria. In this work the predictive capability of COSMO-RS, a predictive model based on unimolecular quantum chemistry calculations, was evaluated for the description of the liquid-liquid equilibria (LLE) and the vapour-liquid equilibria (VLE) of diverse binary mixtures of water and lLs. The effect of the ions conformers on the quality of the predictions was assessed and the quantum chemical COSMO calculation at the BP/TZVP level derived from the lowest energy conformations was adopted. While the LLE predictions degrade with increasing the hydrophilic IL anion character, in general a good qualitative agreement between the model predictions and experimental VLE and LLE data was obtained. COSMO-RS showed to be very helpful as an a prior! predictive method in order to find suitable candidates for a certain task or specific applications before extensive experimental measurements. (C) 2008 Elsevier B.V. All rights reserved.