A novel locally and densely sulfonated dianhydride with four sulfonic acid groups, 1,6,7,12-tetra[4-(sulfonic acid)phenoxylperylene-3,4,9,10-tetracarboxylic dianhydride (SPTDA), was successfully synthesized by direct sulfonation of the parent dianhydride. 1.6,7,12-tetraphenoxyperylene-3,4,9,10-tetracarboxylic dianhydride (PTDA). Sulfonated copolyimides were prepared from SPTDA, nonsulfonated dianhydride 4,4'-binaphthyl-1,1',8,8'-tetracarboxylic dianydride, 4,4'-diaminodiphenyl ether (a) or dodecane-1,12-diamine (b). The synthesized copolymers, with the -SO3H group on the polymer side chain, possess high molecular weights and high viscosities, and they form tough, flexible membranes. The copolymer membrane with an ion exchange capacity of 2.69 mequiv.g(-1) had a proton conductivity of 0.126 S cm(-1) at 20 degrees C and 0.292 S cm(-1) at 100 degrees C; the latter is much higher than that of Nafion (R) 117 under the same conditions. The mechanical properties of the copolymer membranes were almost unchanged after accelerated water stability testing at 140 degrees C for 100 h; this indicates excellent hydrolytic stability of the synthesized copolyimides. (C) 2009 Elsevier B.V. All rights reserved.