Hollow-fiber carbon molecular sieve (CMS) membranes derived from sulfonated poly(phenylene oxide) (SPPO) were studied. Mono-, di- and trivalent metal cations such as Na+, Mg2+, Al3+, Ag+, Cu2+ and Fe3+ were substituted to the proton of sulfonic acid group of SPPO by the ion-exchange method to investigate the effects on gas transport properties for SPPO CMS membranes. In the case of H-SPPO, CMS membranes exhibited higher performance than those of the polymeric precursor, and maximum permeability was obtained when pyrolyzed at 923 K. The introduction of metal cations into SPPO affected the structures of the resulting CMS membranes, of which the Ag- and Cu- form enhanced overall permeabilities. In addition, Fe-SPPO CMS membranes increased He/N-2 and H-2/N-2 permselectivities compared to non-substituted CMS membranes prepared by the same pyrolysis procedure. The highest performance was attained by the Ag-SPPO CMS membrane pyrolyzed at 923 K, where 02 permeability was 178 Barrer and O-2/N-2 permselectivity was 8.7 at 298 K.