Sulfonated polysulfone (SPS) membranes were prepared, and the gas-transport properties of the resulting ionic polymers were examined. Gas-transport measurements were made on dense films of these polymers with a continuous flow technique. The sulfonation of polysulfone and the metal-cation exchange of SPS were confirmed with Fourier transform infrared spectroscopy and electron spectroscopy for chemical analysis. The SPS membranes exchanged with the monovalent metal ions showed higher permeability coefficients than the SPS membranes exchanged with the multivalent metal ions, whereas the selectivities of all the metal-cation-exchanged sulfonated polysulfone (MeSPS) membranes for O-2/N-2 and CO2/N-2 gas pairs were higher than those of SPS membranes. When the MeSPS membranes with metal cations of similar ionic radii were compared, the ideal selectivities of O-2/N-2 and CO2/N-2 through MeSPS with divalent cations were higher than those through MeSPS with monovalent cations.