A family of sulfonated polystyrenes was synthesized and the gas-transport properties of the resulting ionic polymers were investigated. The extent of sulfonation varied from 5 to 27 mol%. Gas-transport measurements were made on either free-standing or supported thin films of these various polymers. Measurements were made on films that contained Na+ as the counterion for the sulfonate groups and on films that contained Mg2+ as the charge-balancing counterion. Permeability coefficients for all gases investigated (O2, N2, CO2, CH4, H2) decreased with increasing extent of sulfonation. Gas-transport selectivity coefficients increased with extent of sulfonation. Selectivity coefficients for the Mg2+-form polymers were higher than for the Na+-form polymers. The best combination of selectivity and permeability was obtained for the Mg2+-form of the most highly-sulfonated polymer (27.50 mol%). This polymer showed an O2/N2 selectivity coefficient of 11.7 with an O2 permeability coefficient of 0.42 barrer.