A new oxygen/nitrogen separation membrane based on an ionomer of sulfonated EPDM (ethylene-propene-diene terpolymer) containing Co2+ ions was synthesized and investigated. It was found that the oxygen permeability coefficient and the separation factor for O-2/N-2 of the ionomer membrane are simultaneously enhanced at lower gas pressure difference. This result is believed to be caused by a polymer coordinate complex formed in the ionomer. ESR spectra provided an evidence of the cobalt-oxygen adduct.