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This paper explores the arguments for using solubility parameters and the regular solution theory for modeling gas solubilities in five different room-temperature ionic liquids (RTILs) at low partial pressures (<1 atm) and low mole fractions (<0.1). The experimentally measured and reported carbon dioxide (CO2) solubilities at low mole fractions (<0.05) suggest positive deviations from Raoult's law for CO2/RTIL solutions. These CO2 solubility deviations from Raoult's law indicate that CO2/RTIL complexations are not the sole controlling factor in relative CO2 solubilities. The RTILs' energies of vaporization and molar volumes appear to be factors in determining relative CO2 solubilities between RTILs. The energies of vaporization for the RTILs were empirically estimated from their melting points.