In this work, tuning the solubility parameter of room-temperature ionic liquids (RTILs) with appended functional groups was explored using a combination of experiment and theory. By predictably altering the solubility parameters of several RTIL solvents, their gas solubility and separation perfon-nance were tailored. This concept was demonstrated by synthesizing and characterizing imidazolium-based RTILs that incorporate nitrile and alkyne functional substituents. The ideal solubility and selectivity values of CO2, N-2, and CH4 at near ambient temperature and pressure were measured for these RTILs. These functionalized RTIL solvents exhibited lower CO2, N-2, and CH4 solubility values but improved CO2/N2 and CO2/CH4 solubility selectivity when compared to analogous nonfunctionalized, n-alkyl-substituted RTILs. A group contribution method was used to predict the solubility parameters of the functionalized RTILs, and these values were used with regular solution theory to predict the solubility and selectivity of the three gases. These predicted gas solubility values were found to be in good agreement with those measured experimentally. Furthermore, the predictions from the group contribution method indicated that inclusion of the nitrile and alkyne functional groups increased the solubility parameter relative to the analogous, n-alkyl-substituted RTILs. These initial results show that the group contribution method offers a valuable guide for systematically designing functionalized RTILs with specific gas solubility and selectivity performance.