Solid, polymer membranes fabricated from room-temperature ionic liquid monomers containing oligo(ethylene glycol) or nitrile-terminated alkyl substituents tethered to imidazolium cations were found to exhibit ideal CO2/N-2 and CO2/CH4 separation factors significantly greater than those with comparable length n-alkyl substituents, with similar CO2 permeability. Polymers containing these functional groups exhibited CO2/N-2 gas separation performance exceeding the "upper bound" of a "Robeson Plot". (C) 2007 Elsevier B.V. All rights reserved.