Significance The gas solubility in 1-dodecyl-3-methylimidazolium [C12MIM] based ionic liquids (ILs) was measured at temperatures (333.2, 353.2, and 373.2) K and pressures up to 60 bar for the first time. The popular UNIFAC-Lei model was successfully extended to long-chain imidazolium-based IL and gas (CO2, CO, and H-2) systems. The free volume theory was used to explain the gas solubility and selectivity in imidazolium-based ILs by calculating the fractional free volume and free volume by the COSMO-RS model. Furthermore, the excess enthalpy of gas-IL system was concerned to provide new insights into temperature dependency of gas (CO2, CO, and H-2) solubility in ILs. The experimental data, calculation, and theoretical analysis presented in this work are important in gas separations with ILs or supported ionic liquid membranes. (c) 2017 American Institute of Chemical Engineers AIChE J, 63: 1792-1798, 2017