The synthesis and characterization of a series of new sulfonated polyimides are described. One-step high temperature polymerization method was used to prepare the sulfonated polyimide copolymers from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), 4,4'-diaminostilbene-2,2'-disulfonic acid (DSDSA), and other three common diamines, i.e., 4,4'-oxydianline (ODA), 4,4'-methylene dianiline (MDA), and 4,4'-(9-fluorenylidene)dianiline (FDA). The sulfonation degree of the copolymers was regulated through variation of the molar ratio of DSDSA to the common diamines. Flexible, transparent, and mechanically strong membranes were successfully obtained. The membranes were characterized with Fourier transform infrared spectra, thermogravimetric analysis, ion-exchange capacity, and proton conductivity measurements. These results showed that the membranes had good thermal stability and exhibited high conductivity at high relative humidity levels, especially in liquid water at 70 degrees C. At high temperatures of above 140 degrees C, the membranes displayed higher conductivities than Nafion (R) membranes because of their high activation energies. (c) 2006 Elsevier B.V. All rights reserved.