Activity coefficients at infinite dilution of 31 organic compounds in three room-temperature ionic liquids (RTILs) were determined using inverse gas chromatography. The measurements were carried out at different temperatures between 313.15 and 333.15 K. The regular solution theory was used to estimate the solubility parameters of each RTIL. Such a theory was also used to predict the CO2 solubility in a given RTIL and to estimate the upper critical solution temperature of a binary system containing 1-butanol and 1-butyl-3-methylimidazolium hexafluorophosphate. The accuracy of the results obtained indicates that the regular solution theory is a good tool to predict the phase behavior of mixtures containing RTILs.