The silver ionic modification approach and dual-layer hollow fiber spinning technology synergistically lead to the significantly enhanced O-2/N-2 and CO2/CH4 selectivity of 9.5 and 118, respectively, with an ultrathin dense-selective layer of 1092 angstrom in the pure gas measurement. Even with the presence of competition in sorption sites between two penetrants, the dual-layer hollow fiber membrane developed in this work still reveals the superior CO2/CH4 selectivity of 87.9 in the mixed gas measurement. To our best knowledge, this is the first time for the polyethersulfone (PES) material to achieve so high CO2/CH4 selectivity through the ionic modification. The electrostatic crosslinking, steric hindrance and affinity to some specific gases (e.g. O-2 and CO2) induced by the incorporation of ionic groups into the PES matrix may be three dominant factors for the advanced gas separation performance in this work. (C) 2010 Elsevier B.V. All rights reserved.