This study focuses on the solubility behaviors of CO2, CH4, and N-2 gases in binary mixtures of imidazolium-based room-temperature ionic liquids (RTILs) using 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C(2)mim] [Tf2N]) and 1-ethyl-3-methylimidazolium tetrafluoroborate ([C(2)mim] [BF4]) at 40 degrees C and low pressures (similar to 1 atm). The mixtures tested were 0, 25, 50, 75, 90, 95, and 100 mol % [C(2)mim][BF4] in [C(2)mim][Tf2N]. Results show that regular solution theory (RST) can be used to describe the gas solubility and selectivity behaviors in RTIL mixtures using an average mixture solubility parameter or an average measured mixture molar volume. Interestingly, the solubility selectivity, defined as the ratio of gas mote fractions in the RTIL mixture, of CO2 with N-2 or CH4 in pure [C(2)mim][BF4] can be enhanced by adding 5 mol % [C(2)mim][Tf2N].