Two isophthalic polyesters synthesized from 4,4'-(9-fluorenylidene) bisphenol, BF, and 4,4'-(1-phenylethylidene) bisphenol, BAP, and a random copolyester made from BF and BAP containing 50 mol % of these aromatic diols were prepared by interfacial polymerization. The resulting polyesters show glass transition temperatures, T-g, above 200 degrees C and onset of decomposition temperatures above 440 degrees C. The values of these thermal properties in the copolyester BAP50/ISO fall between those of the polyesters. It was also found that the thermal stability and maximum on the alpha-transition of the damping factor, tan delta, of BF/ISO were higher than those of BAP/ISO which is an indication of higher rigidity in the former. Wide-angle X-ray diffraction measurements indicated that both the homopolyesters and copolyester were amorphous. The copolyester showed an amorphous pattern with maxima that fell between those of the polyesters. Pure gas permeation measurements in films cast from solution show that gas permeability coefficients increase as the fractional free volume, FFV, of the polyester increases. This is attributed to a lower packing in BF/ISO as compared to BAP/ISO which increases the free volume available for gas permeation. The copolyester shows FFV and permeability coefficients that are intermediate between those exhibited by the homopolyesters. It was also found that ideal gas separation factors for O-2/N-2, CO2/CH4, and CO2/N-2 are higher for BF/ISO which also shows the highest permeability coefficients.