In this work, the liquid-liquid equilibrium (LLE) of fifty ternary systems involving twelve different ionic liquids, comprising 408 experimental tie-lines, was correlated by the UNIQUAC model for the activity coefficient. New UNIQUAC structural parameters rand q for the ionic liquids were determined by quantum chemistry calculations performed with the Gaussian 03 and GAMESS 7.1 packages, including the Density Functional Theory (DFT) for the optimization of the structures and the Polarizable Continuum Method (PCM) for the calculation of molecular areas and volumes. The results, expressed by deviations between experimental and calculated compositions, are very satisfactory, with deviation values about 1.75%. (C) 2009 Elsevier B.V. All rights reserved.