This study focuses on bulk fluid solubility of carbon dioxide (CO2), methane (CH4), hydrogen (H2), and nitrogen (N2) gases in the imidazolium-based RTILs:  1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (emimTf2N), 1-ethyl-3-methylimidazolium tetrafluoroborate (emimBF4), 1-n-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (hmimTf2N), and 1,3-dimethylimidazolium methyl sulfate (mmimMeSO4) as a function of temperature (25, 40, 55, and 70 °C) at near-atmospheric pressures. The experimental behaviors are explained in terms of thermodynamic relationships that account for the negligible vapor pressure of the RTIL as well as the low solubilities of the gases. Results show that, as temperature increases, the solubility of CO2 decreases in all RTILs, the solubility of CH4 remains constant in emimTf2N and hmimTf2N but increases in mmimMeSO4 and emimBF4, and the solubility of N2 and H2 increases. Also, the ideal solubility selectivity (ratio of pure-component solubilities) increases as temperature decreases for CO2/N2, CO2/CH4, and CO2/H2 systems. Experimental values for the enthalpy and entropy of solvation are reported.