Novel random copolyaramides, CTB-75/25, CTB-50/50, CTB-25/75 and PA-100, with different amount of bulky pendant groups, tert-butyl (TERT) and 3,5-bis(trifluoromethyl) benzoyl (BTFB) were synthesized. The polyamides are soluble in organic solvents and show thermal stability up to 400 degrees C (T-g = 272-305 degrees C) as well as mechanical properties with elastic modulus above 1.4 GPa. CTB-25/75 copolyaramide containing 75% BTFB exhibited the highest gas permeability for pure gases (P-CO2 = 134.2 and P-CH4 = 5.86, a = 22.9) and mixed-gas CO2/CH4 (10/90 mol%) ((P-CO2 = 123.81 and P-CH4 = 5.40, alpha = 27.2) at 35 degrees C. CTB-50/50 containing 50% BTFB presented the highest H-2/CH4 and H-2/N-2 permselectivity (alpha(H2/CH4) = 38.3,alpha(H2/N2) = 30.0). Thus, the proper combination of bulky groups will allow to fine-tune gas separation properties into the copolyaramides particularly increasing gas permeability and selectivity of the membranes. This behavior could be attributed to the fact that the incorporation of TERT and BTFB groups not only increase the FFV of polyamide, but also hinder chain rotation, increasing gas selectivity.