A series of soluble copoly(amic acid) precursors were synthesized with varying compositions by the conventional polycondensation of 3,3',4,4'-biphenyltetracarboxylic anhydride (BPDA) and 3,3'-sulfonyldianiline (SDA) using 4,4'-diaminodiphenyl ether-2,2'-disulfonic acid (ODADS), a sulfonated diamine, as a third component ill NMP. The synthesized precursor polymers are soluble in polar aprotic solvents. The chemical structures and inherent viscosities of the copoly(amic acid)s were determined by H-1 NMR spectroscopy and viscometry, respectively. The free standing sulfonated copolyimide membranes were fabricated from solutions of the copoly(amic acid)s by a thermal imidization process and subsequent soaking in sulfuric acid solution. The chemical structure and thermal properties of the sulfonated copolyimides in the films were investigated in detail using FT-IR, DSC, and TGA. The sulfonated copolyimides show mainly three-step thermal degradation, with the degradation behavior being independent of the composition. In addition, the properties of the membranes were investigated, including their water uptake (WU), ion exchange capacity (IEC) and proton conductivity. The WU and IEC values of the sulfonated copolyimides were 8.12-38.0% and 0.18-2.05 meq/g, respectively, depending on the composition of the sulfonated diamine, and the proton conductivity of the copolyimides was dependent on the content of the sulfonated diamine. The proton conductivity values of the membranes were lower than that of Nafion 117, although their IEC values were higher. (C) 2012 Elsevier B.V. All rights reserved.