Two aromatic polyesters and three copolyesters were synthesized by interfacial polycondensation using 4,4'-(hexafluoroisopropylidene)diphenol (HID) and two phthalic dichlorides, isophthaloyl dichloride (ISO) and 5-tertbutyl-isophthaloyl dichloride (TERT). The polymers obtained were soluble in common chlorinated solvents. The properties of these aromatic polyesters and copolyesters were characterized by FTIR, density, inherent viscosity, TGA, and DSC. Thermal properties such as glass transition temperature, onset of decomposition, and thermal stability of the homopolymer, poly(hexafluoroisopropilydene)5-tertbutylisoplithalate (HFD/TERT), were higher than those of homopolymer poly(hexafluoroisopropilydene)isophthalate (HFD/ISO). Thermal properties of the copolyesters HFD/TERT-co-HFD/ISO depend upon the amounts of the tertbutyl group HFD/TERT, present in the copolymer. Gas permeability coefficients of all polyarylates were measured at 35 degrees C. The effect of different concentrations of the bulky tertbutyl group at the 5-position in the isophthaloyl moiety on He, O-2, N-2, and CO2 permeability, diffusion, and solubility coefficients were determined. Gas permeability and diffusivity increase as the concentration of TERT moiety increases in the copolymers. The results indicate that polymers containing the largest amounts of the bulky lateral tertbutyl group show the highest gas permeability. The increment in gas permeability and diffusivity produces a decrease in selectivity, which is attributed to the effect of the large pendant tertbutyl groups in the aromatic polyesters and copolyesters, which decrease the chain packing efficiency and induce a larger fractional free volume. (c) 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2207-2216, 2007