Polysulfone based on bisphenol A has been extensively used as a material for membrane-based gas separation. For polymers with aromatic rings in their backbones, such as polysulfones and polyimides, changing the connecting bond positions from meta to para seems to increase their permeability and diffusivity to gases. Cavity size distributions of three isomer pairs (PSF and 3,4'-PSF, PSF-P and PSF-M, 6FDA-6FpDA and 6FDA-6FmDA) are calculated through molecular simulation. The diffusivity of small molecules in these isomers is also obtained by molecular dynamics. For all three isomer pairs, the average cavity size in para isomers is larger than in meta isomers. The molecular dynamics determined diffusion coefficients of neon in para isomers are also larger than in meta isomers. These results are consistent with the experimental observation that room-temperature gas diffusion in para isomers is faster than in meta isomers. (c) 2005 Elsevier Ltd. All rights reserved.