Three isomeric aromatic polyamides were synthesized using as a basis 4,4'-(9-fluorenylidene) diamine and different phtalic diacid chlorides. The influence of structural changes on polyamide properties when the phthalic moiety changed from para linked, poly(fluorenylidene terephthaloyl amide), BFA/TERE, to meta linked, poly(fluorenylidene isophthaloyl amide), BFA/ISO, phenyl substitution or meta linked phenyl substitution with a large pendant group, poly(fluorenylidene tertbutyl isophthaloyl amide), BFA/TERT, on these polyamides were assessed. Thermal properties measurements indicate that BFA/TERE has the highest thermal decomposition temperature above 450degrees and Tg, while BFA/ISO and BFA/TERT have thermal properties that are only slightly lower. Gas permeability coefficients of the three isomeric aromatic polyamides were measured at different temperatures. The results indicate that BFA/TERT has the largest permeability and diffusion coefficients while BFA/ISO shows the lowest. These results are attributed to a larger fractional free volume in the former compared to the later. Permeability coefficients for all gases tested show an Arrhenius type behavior with temperature for these aromatic polyamides.