Gas permeability coefficients for poly (dimethylsiloxane) (PDMS) and a series of polyphosphazene (PPh) materials are measured before and after 30 W argon radio frequency (RF)-plasma surface treatments. The gas transport properties of the entire series of PPh materials are relatively unaffected by the plasma treatments. In contrast with the PPhs, the same plasma treatment conditions lead to 40-80% reductions in PDMS oxygen permeability coefficients. The decreases in PDMS permeability coefficients are accompanied by occasional increases in gas pair selectivity, however, many samples show no such increase. Following plasma treatment, the highest PDMS O-2/N-2 selectivity observed is 3.1 with an 80% decrease in oxygen permeability. In cases where enhanced selectivity is observed, the activation energy of permeation in PDMS is shown to increase significantly.