Novel membranes based on sulfonated poly (phenylene oxide) (SPPO) was developed. SPPO membranes in the hydrogen form were converted to metal ion forms. The effect of exchange with metal ions including monovalent (Li+, Na+, K+), divalent (Mg2+, Ba2+, Ca2+) and trivalent (Al3+) ions was investigated in terms of permeation rate and permeation rate ratios for CO2 and CH4 gases. Both dense homogeneous membranes and thin-film composite (TFC) membranes were studied for their gas separation characteristics. The effect of membrane preparation conditions and operating parameters on the membrane performance were also investigated. The selectivity of the TFC membrane increased as the cationic charge density increased as a result of electrostatic cross-linking. TFC membrane of very high selectivity was achieved by coating a thin layer of SPPO-Mg on a PES substrate. (C) 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 735-742, 2000.