The modified UNIFAC-Lei model (Mod. UNIFAC-Lei) is proposed for the first time for ionic liquid (IL)-methane (CH4) systems over a wide temperature and pressure range. The group binary interaction parameters were obtained by correlating the experimental solubility data of CH4, both from the literature and that measured in this work. The CH4 solubility in common pure ILs and mixed ILs at temperatures down to 243.15 K was measured to determine unknown solubility data at low temperatures. The Mod. UNIFAC-Lei model can be used to predict CH 4 solubility in both pure and mixed ILs, at either high or low temperatures. Moreover, it is interesting to find that cations and anions in complementary IL pairs ([C-1] [A(1)] + [C-2] [A(2)] or [C-2] [A(1)] + [C-1] [A(2)]) with equimolar amounts can be freely exchanged. A low temperature is favorable for increasing both the solubility of CH4 and the selectivity of CH4 to CO2 in ILs.