Polyethylene glycol (PEG) is grafted onto ionic polymers poly [p-vinylbenzyltrimethylammonium tetralluoroborate] (PLVBTMA][BF4]) and poly[2-(methylacryloyloxy)ethyl-trimethylammoniumtetrafluoroborate] (P[MATMAI[BF4]). Membranes made of P[VBTMA][BF4]-g-PEG and P[MATMA][BF4]-g-PEG are found to be CO2 selective for CO2/CH4 and CO2/N-2 separations, and less brittle than those made of pure P[VBTMA][BF4] and P[MATMA][BF4]. At the same permeability, the P[MATMA][BF4]-g-PEG membranes are demonstrated to have a higher CO2 selectivity than the previous polymeric membranes. The selectivity of both P[VBTMA][BF4]-g-PEG and P[MATMA][BF4]-g-PEG is primarily due to the solubility differences, not the diffusivity differences. (c) 2006 Elsevier B.V. All rights reserved.