The synthesis, thermal, and gas transport properties of poly(hexafluoroisopropylidene isophthalamide), HFA/ISO homopolymer, and HFA/TERT-co-HFA/ISO copolyamides with different poly(hexafluoroisopropilydene-5-t-butylisophthalamide), HFA/TERT, ratios are reported. The results indicate that the glass transition temperatures of the copolyamides increase as the concentration of HFA/TERT in the polyamide increases. The gas permeability coefficients in the polyamides and copolyamides are independent of pressure or decrease slightly particularly with CO2, N-2, and CH4. It was seen that HFA/TERT is 2-6 times more permeable than HFA/ISO, depending on the gas being considered. This was assigned to the presence of the bulky lateral substituent, t-butyl group in HFA/TERT and HFA/TERT-co-HFA/ISO copolyamides. This substituent increases fractional free-volume, as expected. Therefore, the gas permeability and diffusion coefficients generally increase with increasing fractional free-volume. The experimental results for the gas permeability and permselectivity for the copolyamides was well represented by a logarithmic mixing rule of the homopolyamides permeability coefficients and their volume fraction. The selectivity of gas pairs, such as O-2/N-2, CO2/CH4, and N-2/CH4 decreased slightly with the addition of HFA/TERT. The temperature dependence of permeability for homopolyamides and copolyamides can be described by an Arrhenius type equation. (c) 2005 Wiley Periodicals, Inc.