Dispersing highly selective rigid "inserts" such as zeolites and carbons into polymers can produce novel membrane materials with improved gas mixture permselectivity. Rigid inserts frequently cause the permeability in an "interphase" surrounding individual inserts to differ markedly from the intrinsic permeability of the matrix polymer. While existing theoretical models describe permselectivity reasonably well, they fail to predict the absolute permeabilities for individual components in such hybrid media. In fact, the degree of deviation from the simple model predictions provides insight into the detailed properties of the interphase, which has been neglected in previous analyses of these hybrid materials. Extension of an existing model provides a framework to analyze gas permeation properties of model systems with rigid molecular sieve inserts in glassy matrices. Inhibited segmental motion due to surface attachment and matrix contraction at the interface during preparation of these materials is hypothesized as the key cause of the observed behavior. (C) 2004 American Institute of Chemical Engineers.