A series of poly(ether imide)s are investigated using positron annihilation lifetime spectra [PALS] and gas permeation measurements. The glass transition temperature T-g,T- determined by differential scanning calorimetry, does not correlate with the void sizes calculated from density or determined by PALS, implying that in these materials polymer-polymer chain interactions and not free volume has a dominant role in determining the overall chain dynamics. Significant differences observed in the permeability process with respect to carbon dioxide and nitrogen, highlight the role of polar interactions between the diffusing gas molecule and the polymer matrix on the transport processes. Diffusion coefficients do not change in a simple manner with void size. The solubility parameters vary across the series of materials and produce a small variation in the solubility coefficients for nitrogen and argon, but significantly differences were observed for carbon dioxide, reflecting the influence of polar interactions plasticizing the matrix. This study indicates that whilst the void structure is important in influencing gas transport, chain-chain interactions are controlling many of the higher temperature physical properties of these polymers. High values of selectivity between carbon dioxide and nitrogen were observed indicating that these materials have potential applications as gas separation membranes. POLYM. ENG. SCI., 56:427-434, 2016. (c) 2016 Society of Plastics Engineers