Room temperature ionic liquids (RTILs) can be used for many applications where solubility is one of the main parameters. This paper discusses the use of regular solution theory to model gas solubility at low pressure with empirically determined constants that depend only on the gas. The gases investigated were ethane, ethene, propane, propene, isobutane, butane, 1-butene, and 1,3-butadiene. It was found that the solubility increased as the number of carbons increased, and there was also an increase in solubility for the hydrocarbons with the same carbon number when the number of carbon double bonds increased. The RTILs used in this study were butylmethylimidazolium hexafluorophosphate ([bmim] [PF6]), butylmethylimidazolium tetrafluoroborate ([bmim] [BF4]), ethylmethylimidazolium bis(trifluoromethanesulfonyl)amide ([emim] [Tf2N]), ethylmethylimidazolium trifluoromethanesulfone ([emim] [CF3SO3]), and ethylmethylimidazolium dicyanamide ([emim] [dca]).