This paper examines the dependence of gas solubility on the surface tension of room-temperature ionic liquids (RTILs). The solubility phenomena of the CO2, ethylene, propylene, I-butene, and 1,3-butadiene in five imidazolium-, four phosphonium-, and eight ammonium-based RTILs are explained using the Hansen RTIL solubility parameters that were calculated from surface tension measurements. The Hansen-solubility parameters were estimated from the surface tensions of the respective RTILs with a proportionality constant that is a function of the ratio of the number of nearest-neighbor interactions in the bulk liquid to those on the surface. The calculated solubility parameters are almost independent of temperature. The surface tension solubility parameter model explains the effect of variation of alkyl chain lengths in the cations and the variation of the RTIL anions on gas solubility within an error of +/-15%. The analysis may indicate that the CO2-anion interaction is not the dominant factor that determines the relative CO2 solubility between RTILs.