Structure/permeability correlations for a family of aromatic polyamide-imides are presented. The variations in chemical structure lead to significant changes in permeability. The crystallization of PAP results in a very low permeability. The introduction of bulky group tends to increase permeability without a corresponding decrease in permselectivity. This result contributes to the inhibition of chain packing and segmental mobility. Wide-angle X-ray diffraction measurements of average segmental spacing of the materials and fractional free volume calculations characterize the packing of the different polymers.