This paper describes the results of positron annihilation lifetime measurements on a series of poly(ether imide)s for which permeabilities and permselectivities for gas separation have been determined previously. Positron lifetimes and intensities vary systematically with changes in polymer structure and correlate with the permeabilities in separation of carbon dioxide and methane. Some apparent scatter in the data might, in part, be associated with differences in the interactions of ortho-positronium with molecular cavities as polymer structures are varied.