Supported ionic liquid membranes (SILMs) were prepared by impregnating porous anodic alumina membranes with seven different ionic liquids, with different alkyl imidazolium cations and bis(trifluoromethylsulfonyl)imide, acetate and trifluoroacetate anions. The permeability of these membranes to carbon dioxide and nitrogen was measured, yielding CO2 permeabilities greater than 1000 Barrer and ideal CO2/N-2 selectivities between 12 and 21, values that lie close to the Robeson upper bound for this separation. A comparison of these results to values reported by others for other SILMs and for polymerized SILM systems shows larger CO2 permeabilities and generally comparable CO2/N-2 selectivities. Measured CO2 permeance values were also found to be larger than permeance values predicted from bulk CO2 solubility and diffusivity values that have been previously reported by our laboratory. In addition, permeance values measured with support membranes with 100 nm nominal pore size were larger than values measured with membranes prepared with 20 nm nominal pore size support membranes. We hypothesize that these differences result from the influence of ionic liquid-support membrane interactions on SILM performance. (C) 2011 Elsevier B. V. All rights reserved.