Trimethylsilyl (TMS) groups were introduced with controlled degree of substitution (DS) onto the phenylene rings of various polysulfones. The introduction of TMS groups resulted in a marked increase in oxygen permeabilities with small concurrent decreases in oxygen/nitrogen permselectivities. Although TMS groups are bulky, they are highly mobile and are expected to reduce chain packing as evidenced by larger specific volumes and d-spacings with increasing DS. The higher is DS, the greater the reduction in the chain packing that occurs. Dynamic mechanical analyses of sub-glass-transition relaxation, i.e., gamma-relaxation behavior, showed that the TMS groups affected the local chain motion. In particular, the motion of unsubstituted phenylene rings increases with DS. Therefore, both the loosened chain packing and the increased local motion by substitution of TMS may result in the increase in the gas permeability. (C) 2003 Elsevier B.V. All rights reserved.