Novel imidazolium-based room-temperature ionic liquids (RTILs) with one, two, or three oligo(ethylene glycol) substituents were synthesized. Solubilities and ideal solubility selectivities of CO2, N-2, and CH4 at low pressure (1 atm) in these RTILs were determined using a pressure decay technique. Comparison to corresponding alkyl analogues of these RTILs reveals similar levels of CO2 solubility but lower solubilities of N-2 and CH4. As a consequence, RTILs with oligo(ethylene glycol) substituents were observed to have 30-75% higher ideal solubility selectivities for CO2/N-2 and CO2/CH4.