The synthesis, thermal, and gas transport properties of poly(benzophenone isophthalamide), DBF/ISO, poly(benzophenone-5-tert-butylisophthalamide), DBF/TERT, homopolymers, and their copolyamides with different DBF/TERT ratios are reported. The results indicate that the glass transition temperatures of the copolyamides increase as the concentration of DBF/TERT in the polyamide increases. The gas permeability coefficients for DBF/ISO are around 10(-2) Barrers for O-2 which situates this polymer as a barrier polymer. It was also found that permeability coefficients in all polyamides and copolyamides are independent of pressure for He or decrease slightly particularly with O-2, CO2, and N-2. It was seen that DBF/TERT is up to 15 times more permeable than DBF/ISO, depending on the gas being considered. This behavior was assigned to the presence of the bulky lateral substituent, the tert-butyl group, in DBF/TERT and DBF/TERT-co-DBF/ISO copolyamides. This bulky substituent increases fractional free volume and interchain spacing; as a consequence, the gas permeability and diffusion coefficients generally increase. The experimental results for the gas permeability coefficients and permselectivity for the copolyamides was well represented by a semilogarithmic mixing rule of the homopolyamides permeability coefficients as a function of their volume fraction. The selectivity of gas pairs, such as He/O-2 and He/CO2, decreased slightly with the addition of DBF/TERT The temperature dependence of permeability for homopolyamides and copolyamides can be described by an Arrhenius type equation. (c) 2007 Wiley Periodicals, Inc.