Interesting properties of ionic liquids lead to their application as sub-component of membrane structures. Supported ionic liquid membranes (SILMs) are porous membranes whose pores are saturated with ionic liquids. Two ionic liquids, pr[mim](2)[Tf2N](2)[1,3-di(3-methyl-imidazolium) propane bin (trifluoromethylsulionyl) imide] and, h[mim](2) [Tf2N](2) [(1,6-di(3-methylimidazolium)hexane bis(trifluoromethylsulionyl)imide)], were synthesized in our laboratory and stabilized on an alumina porous support. Permeability and permselectivity of carbon dioxide and methane using membranes containing these ionic liquids were then measured. The experiments were performed in the pressure range of 1050 kPa and temperature of 300-320 K. Influences of temperature and pressure on the permeability and selectivity of gases have been investigated. The obtained results were compared with those of [hmim] [Tf2N] (1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imicle) ionic liquid, which was investigated in our previous work. The results showed these two ionic liquids show smaller values for carbon dioxide and methane permeability for CO2/CH4 separation, compared with [hmim][Tf2N]. Moreover, dependence of the carbon dioxide permeability on the viscosity of ionic liquids considering investigated ionic liquids in this study and those of our previous studies on two imidazolium-based ionic liquids, [hmim][Tf2N] and [bmim][PF6] has been correlated in the range of operating temperatures. (C) 2013 Elsevier B.V. All rights reserved