An analytic version of statistical associating fluid theory (SAFT), namely extended SAFT-BACK equation of state has been developed to predict the pressure-volume-temperature (pVT) and second-order thermodynamic properties of pure alkyl-imidazolium ionic liquids (ILs) with [PF6](-) and [BF4](-) anions. The new molecular parameters are obtained by fitting to reliable density data of pure ILs with a fitting error of 0.12% on average. The ionic liquids composed of chain-like dipolar non-spherical ion-pairs and the extended SAFT-BACK model are found to be able to correctly describe the speed of sound and isothermal compressibility as well as pVT properties of ionic liquids studied in this work. (C) 2013 Elsevier B.V. All rights reserved.