A novel series of aromatic polyamides was synthesized by postpolymerization modification using a simple strategy to introduce different bulky pendant groups into a precursor polymer. The polyamides possess high thermostability with Tg values in the range of 298-312 degrees C and good mechanical properties. These polyamide membranes also exhibit enhanced gas separation performance, especially for CO, and H-2 separations, with plasticization resistance in the range tested (2.02-10.13 bar). The most remarkable result for gas permeability is attained with P3 bearing 4-tert-butylbenzoyl (TBB) as pendant group P-CO2 = (88.1, P-H2 = 101 with selectivity (a) for CO2/CH4 = 19.2 and H-2/, N-2 = 21.5). The highest permselectivity is obtained for P2 containing p-toluoyl (p-T) as pendant group (CO2/CH4 = 26.1 and H-2/CH4 = 38.1).