Directly copolymerized wholly aromatic sulfonated poly(arylene ether sulfone) copolymers derived from 4,4'-biphenol, 4,4'-dichlorodiphenyl sulfone, 3,3'-disulfonated, and 4,4'-dichlorodiphenyl sulfone (BPSH) were evaluated as proton-exchange membranes for elevated temperature operation (100-140 degreesC). Acidification of the copolymer from the sulfonated form after the nucleophilic step (condensation) copolymerization involved either immersing the solvent-cast membrane in sulfuric acid at 30 degreesC for 24 h and washing with water at 30 degreesC for 24 h (method 1) or immersion in sulfuric acid at 100 degreesC for 2 h followed by similar water treatment at 100 degreesC for 2 h (method 2). The fully hydrated BPSH membranes treated by method 2 exhibited higher proton conductivity, greater water absorption, and less temperature dependence on proton conductivity as compared with the membranes acidified at 30 degreesC. In contrast, the conductivity and water absorption of a control perfluorosulfonic acid copolymer (Nafion 1135) were invariant with treatment temperature; however, the conductivity of the Nafion membranes at elevated temperature was strongly dependent on heating rate or temperature. Tapping-mode atomic force microscope results demonstrated that all of the membranes exposed to high-temperature conditions underwent an irreversible change of the ionic domain microstructure, the extent of which depended on the concentration of sulfonic acid sites in the BPSH system. The effect of aging membranes based on BPSH and Nafion at elevated temperature on proton conductivity is also discussed. (C) 2003 Wiley Periodicals, Inc.