In this work, the ion-specific electrolyte perturbed-chain statistical associating fluid theory (ePC-SAFT) was extended to predict the second-order thermodynamic derivative properties and gas solubility of the ionic liquids (ILs) containing one of the IL cations ([C n mim]+, [C n py]+, [C n mpy]+, [C n mpyr]+, and [THTDP]+) and one of the IL anions ([Tf2N]−, [PF6]−, [BF4]−, [tfo]−, [DCA]−, [SCN]−, [C1SO4]−, [C2SO4]−, [eFAP]−, Cl–, [Ac]−, and Br–). The ideal-gas isobaric heat capacities of ILs were estimated by the group contribution method for obtaining the heat capacity. The model prediction results were compared with the available experimental data, and the comparison shows that the ePC-SAFT prediction is reliable for most ILs. Furthermore, one adjustable ion-specific binary interaction parameter between the IL ion and CO2 can be used to further improve the model prediction performance for the CO2 solubility in ILs.