This paper shows how gas solubility can be predicted in Rmim-based room-temperature ionic liquids (RTILs). We have developed a model to illustrate that ideal, single-gas solubility in Rmim-based RTILs is primarily a function of molar volume of the RTIL. This method is used to estimate bulk-fluid gas solubility, showing that for Rmim-based RTILs there is a particular molar volume where a maximum amount of gas per volume of RTIL occurs at a given pressure and temperature. This method is also used to estimate gas permeability and gas pair separation selectivity for ideal CO2/N-2 and CO2/CH4 separations. A comparison to traditional polymer membranes utilized in these separations is included in the form of a "Robeson plot".