Poly(arylene ether sulfone)-based ionomers containing sulfofluorenyl groups have been synthesized for applications to polymer electrolyte membrane fuel cells (PEMFCs). In order to achieve high proton conductivity and chemical, mechanical, and dimensional stability, the molecular structure of the ionomers has been optimized. Tough, flexible, and transparent membranes were obtained from a series of modified ionomers containing methyl groups with the ion-exchange capacity (IEC) ranging from 1.32 to 3.26 meq/g. Isopropylidene tetramethylbiphenylene moieties were more effective than the methyl-substituted fluorenyl groups in giving a high-IEC ionomer membrane with substantial stability to hydrolysis and oxidation. Dimensional stability was significantly improved for the methyl-substituted ionomer membranes compared to that of the non-methylated ones. This new ionomer membrane showed comparable proton conductivity to that of the perfluorinated ionomer membrane (Nafion 112) under a wide range of conditions (80-120 degrees C and 20-93% relative humidity (RH)). The highest proton conductivity of 0.3 S/cm was obtained at 80 degrees C and 93% RH. Although there is a decline of proton conductivity with time, after 10 000 h the proton conductivities were still at acceptable levels for fuel cell operation. The membranes retained their strength, flexibility, and high molecular weight after 10 000 h. Microscopic analyses revealed well-connected ionic clusters for the high-IEC membrane. A fuel cell operated using the polyether ionomer membrane showed better performance than that of Nafion at a low humidity of 20% RH and high temperature of 90 degrees C. Unlike the other hydrocarbon ionomers, the present membrane showed a lower resistance than expected from its conductivity, indicating superior water-holding capability at high temperature and low humidity.