Blends of polyimide isomers containing hexafluoroisopropylidene in the central moiety of the diamine residue have been studied. The isomers differed by having either a meta or a para linkage between the diamine and dianhydride residues. The miscibility of these materials was investigated by studying the glass transition temperature behavior using differential scanning calorimetry. Mixtures of isomer pairs, such as 6FDA–6FmDA and 6FDA–6FpDA, exhibited one glass transition temperature. Tg, and were therefore miscible. Mixtures of nonisomer pairs exhibited two Tg's and were immiscible. The gas sorption and transport properties of the blends of the 6FDA–6FmDA and 6FDA–6FpDA isomers were characterized for a variety of gases at 35°C for pressures up to 60 atm. The permeabilities and permselectivities in the miscible blends fell between those of the pure components and were approximately logarithmic averages of the pure component properties. The miscibility of the polyimide isomers enables one to tailor the composition of the material to optimize the gas separation and mechanical properties. © 1993 John Wiley & Sons, Inc.