A new series of poly(arylene ether) ionomers containing sulfofluorenyl groups has been synthesized for the applications to polymer electrolyte fuel cells (PEFCs). The parent polyethers were regioselectively substituted with sulfonic acid groups to give the ionomers with the ion exchange capacity up to 2.0 mequiv/g. The ionomers 2a (homopolymer) and 2b (copolymer) gave a tough, flexible, ductile, and transparent membrane by casting from the solution. Thermal stability with decomposition temperature of ca. 250 degrees C and no glass transition temperature was confirmed for the ionomers. The membrane of 2a (ion-exchange capacity (IEC) = 1.80 mequiv/g) showed outstanding stability to oxidation and hydrolysis as confirmed in the accelerated testing in hot Fenton's reagent or in the atmosphere at 140 degrees C and 100% relative humidity. The membrane of 2a (IEC = 1.63 mequiv/g) exhibited better mechanical properties (lower elongation and higher strength) than those of Nafion 117. The fully hydrated membrane of 2a (IEC = 1.14 meq/g) showed the highest proton conductivity (0.2 S cm(-1)) at 140 degrees C. The proton conductivity was durable over 1300 h. Lower gas (H-2 and O-2) permeation was confirmed for 2a rather than for Nafion. All those properties of the ionomers 2 are promising as an electrolyte membrane for high-temperature PEFCs.