In this paper, the Valderrama and Robles group contribution model for the critical properties is extended to the prediction of densities of ionic liquids (ILs) at varying temperatures and pressures, where the critical properties of ILs are represented by the modified Lydersen-Joback-Reid group contribution method, and the density is predicted by virtue of the Patel-Teja (PT) equation of state (ECG). The group increments for totally 47 groups with respect to the extended Group Contribution Patel-Teja (GC-PT) model were determined on the basis of experimental density data for 747 pure ILs at atmospheric pressure and ambient temperature. The group increments are suitable for both the GC-PT model and the original Valderrama and Robles model for the density prediction of ILs. The correlation accuracy in terms of overall average absolute relative deviation (AARD) is 4.4% for 918 data points of density at ambient temperature and atmospheric pressure. The applicability of the GC-PT model is justified by predicting the densities of imidazolium-, pyridinium-, and phosphonium-based ILs containing various anions over a wide range of temperatures and pressures and the vapor pressures of five alkylimidazolium-based ILs at varying temperatures, which implied the rationality of the group increments and the critical properties of ILs, as well as the potential uses of the GC-PT model for the thermodynamic modeling of IL-containing systems. (C) 2011 Elsevier Ltd. All rights reserved.