A novel polymer electrolyte membrane (PEM) based on sulfonated poly(aromatic imide-co-aliphatic imide) (S-coPI) is synthesized by the polycondensation of 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BPTDA) with 4,4'-diaminodiphenylmethane (DDM), 4,4'-diaminodiphenylmethane-3,3'-disulfonic acid disodium salt (S-DDM), and adipic acid dihydrazide (ADH). The ADH is utilized to increase the flexibility of the backbone structure due to its aliphatic structure. The sulfonation degree of S-coPI is controlled by varying the mole ratio of DDM, S-DDM, and ADH. The S-DDM containing two sulfonated groups is confirmed by H-1-NMR. S-coPI membranes are characterized by FT-IR to identify the characteristic peaks of the sulfonated group at 1,298, 1,159, 1,098, and 1,065 cm(-1). Thermogravimetric analysis is used to investigate the thermal stability of S-coPI; it undergoes 2 steps in the degradation process, one at 320 A degrees C which represents the decomposition of the sulfonated group and one at 560 A degrees C which represents the polymer backbone degradation. The S-coPI-3, with the highest sulfonation degree, possesses proton conductivity value of about 1.16 x 10(-2) S cm(-1). The methanol permeability of the S-coPI films are in the range of 3.864 x 10(-8)-6.521 x 10(-8) cm(2) s(-1), about 26-40 times lower than that of Nafion 117.