A series of four imidazolium salts containing increasing lengths of fluoroalkyl substituents was synthesized. The three that exist as molten salts at 296 K were tested for their gas separation properties relating to CO 2, O 2, N 2 and CH 4 using a supported ionic liquid membrane (SILM) configuration. These fluoroalkyl-functionalized room-temperature ionic liquids (RTILs) were found to exhibit ideal selectivities for CO 2/N 2 separation that were lower than their alkyl-functionalized analogues, but higher ideal selectivity for CO 2/CH 4 separation. The differences in performance of fluoroalkyl-functionalized RTILs relative to their alkyl-functionalized counterparts are explained through the use of solubility parameters, group contributions and in context of the classically observed deviations of fluorocarbons from “regular” solution behavior.