An improvement in gas permeation properties has been obtained by introducing bulky silyl side groups at sites ortho to either the ether or sulfone Linkage of polysulfone and poly(phenyl sulfone) chains. The sizes of the substituents were systematically increased from trimethylsilyl, to dimethylphenylsilyl, and to diphenylmethylsilyl, and the degree of substitution was also controlled. The sterically smallest trimethylsilyl group substituted at the ortho ether site markedly increased the oxygen permeability from 1.1 to 7.1 barrers compared with that of unmodified polysulfone at a small tradeoff in oxygen/nitrogen selectivity. Unexpectedly, the oxygen permeability remained almost unchanged from polysulfone with the introduction of more bulky dimethylphenylsilyl and diphenylmethylsilyl substituents. It was also found that substitution at the sites ortho to ether linkage was more effective than the sulfone linkage in improving transport properties. The improved gas separation properties appear to be associated with increased free volume, evidenced from the increased d-spacing and specific volume. Further, the mobility of pendant substituents, characterized by dynamic mechanical analysis, correlated strongly with the d-spacing and specific volume. This leads us to the conclusion that it is substituent mobility rather than the size of the pendant side group that is the critical factor in determining free volume and transport properties.