The permeability coefficients for H-2, O-2, and N-2 in four polyimides and one polypyrrolone based on 2,2’-bis(p-trimellitoxyphenyl)hexafluoropropane dianhydride were determined at 30 degrees C and 1 atm. Systematic variations in chemical structure were found to lead to significant changes in permeabilities and selectivities. The relationship between the chemical structure of the polymers, some of their physical properties (d-spacing, fractional free volume), and their gas permeability behavior are discussed. Introduction of 2 flexible segments tends to increase permeability but decrease selectivity. Structural moieties, which simultaneously inhibit chain packing and intrasegmental mobility, may increase permeability while maintaining selectivity. Temperature dependency of gas permeabilities was also reported for polypyrrolone.