This article demonstrates that transport of gases through glassy polymers is significantly influenced not only by the absolute amount but also by the distribution of free volume. Two stereoisomers of polynorbornene with nearly equivalent total free volume, but markedly different average free-volume sizes, were evaluated. The free-volume element size was probed with positron annihilation lifetime spectroscopy, wide-angle X-ray scattering, gas sorption, and molecular modeling. The permeation, sorption, and diffusion of light gases were measured in each stereoisomer at 35 degreesC. All analytical techniques indicated that one isomer (labeled as Architecture II) had a larger average free-volume element size but fewer elements. This isomer also had a very slightly higher bulk density (1.000 vs 0.992 g/cm(3) for the other stereoisomer). Architecture II also had gas sorption and diffusion coefficients that were two to three times those of the less dense counterpart. These differences have been attributed to differences in the free-volume element size available within the polymer matrix. (C) 2003 Wiley Periodicals, Inc.