Conformational parameters and free, occupied, accessible, and Van der Waals volume values for a group of polymers containing phenylquinoxaline rings have been calculated using an original method and have been correlated with the data regarding glass transition temperature and transport parameters given in the literature. Thus, it was shown that conformational rigidity determines the packing of macromolecules in solid state, and therefore the free volume and glass transition temperature. The introduction of phenylquinoxaline rings in polyimides leads to a decrease of selectivity to all gases due to the increase of free volume in comparison with initial polyimides. But it leads to the increase of selectivity of the gas pair H2/G, where G is any simple gas because of an increase of the number of small microcavities. (C) 2008 Wiley Periodicals, Inc.