A series of four imidazolium salts containing increasing lengths of fluoroalkyl substituents was synthesized. The three that exist as molten salts at 296K were tested for their gas separation properties relating to CO2, O-2. N-2 and CH4 using a supported ionic liquid membrane (SILM) configuration. These fluoroalkyl-functionalized room-temperature ionic liquids (RTILs) were found to exhibit ideal selectivities for CO2/N-2 separation that were lower than their alkyl-functionalized analogues, but higher ideal selectivity for CO2/CH4 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. (C) 2008 Elsevier B.V. All rights reserved.