In this work, the Group Contribution Equation of State (GC-EoS) is extended to model two new ionic liquid families, namely, 1-alkyl-3-methylimidazolium dimethylphosphate ([-Melm](+)[Me2PO4](-)) and 1-alkyl-3-methylimidazolium diethylphosphate ([-Melm](+)[Et2PO4](-)). Pure group parameters for the functional groups [-Melm](+)[Me2PO4](-) and [-Melm](+)[Et2PO4](-) and interaction parameters between these groups and the main paraffinic and aromatic subgroups (CH3, CH2, ACH, AC, ACCH(3), CH2=CH) were estimated using infinite dilution activity coefficients from literature. In order to evaluate the performance of the new parameters, the model was compared with experimental data on the phase behavior of binary mixtures, showing an average relative deviation of 4.5%. (C) 2018 Elsevier B.V. All rights reserved.