A series of aromatic polyimides have been investigated for their permeation properties to oxygen, nitrogen, helium, carbon dioxide and methane. The polymers are soluble, film-forming polyimides based on new anhydride monomers containing carbonyl groups as connecting linkages of phenyl rings and bulky side groups like phenyl and t-butyl. To assist in explaining the experimental results, molecular modeling was performed to calculate density, free volume and chain parameters that could account for the behavior of the polymers as selective barriers for gas penetrants. High values of O-2/N-2 selectivity and good permeabilites were observed for some polymers; their properties lie near the upper bound for this gas pair. Gas permeability typically increased with increasing free volume, and, in general, free volume could be related to the chemical composition of the polymer backbone and to the nature of the pendent groups. (C) 2003 Elsevier Science B.V. All rights reserved.