The permeability of six different polypyrrolone membranes to H-2, O2 and N2 was determined at 30-degrees-C and 1 atm. The variations in chemical structure were found to lead to significant changes in permeabilities and selectivities. Substitution of 6FDA for PMDA in polypyrrolone appeared to result in a significant increase in gas permeability and a correspondingly large decrease in selectivity. Further analysis showed that permeability and selectivity changes in these cases were primarily due to the alteration of diffusivity factors. The relationship between gas separation properties and chemical structure is discussed in terms of intrasegmental mobility and intersegmental packing of the constituent polymers. The average segmental spacing of the materials measured by a wide-angle X-ray diffraction characterized the packing of these different polymers.