High-performance sulfonated polyarylenethioethersulfone (SPTES) polymers have been developed as membranes for fuel cells. These high-molecular-weight polymers synthesized by a polycondensation process have an aromatic backbone along with high sulfonic acid content that provides for their high conductivity and robust mechanical properties. Bulky phenyl-based endcapping agents are incorporated into the system to maintain high water stability and retain high proton conductivity. Films with good mechanical properties were obtained by solvent casting. SPTES polymer systems with a 50% degree of sulfonation (SPTES-50) exhibited high proton conductivity (> 100 mS/cm) at 65 degrees C and 85% relative humidity. Membrane electrode assemblies (MEAs) fabricated using SPTES-50 electrolytes that incorporate conventional electrode application techniques have shown high proton mobility. Electrochemical evaluation was performed using nonlinear regression analysis to obtain Tafel parameters. The electrochemical performance of SPTES-50 was comparable to Nafion. Electrochemical impedance spectra were analyzed in terms of a pore-diffusion model. Catalyst utilization for SPTES MEAs using conventional electrode inks with perfluorinated binders was similar to that exhibited by Nafion. Estimates of hydrogen-fuel permeability based upon measured open-circuit voltage indicate that SPTES-50 MEAs exhibit a slightly higher rate of fuel crossover compared to Nafion. Thermogravimetric analysis shows good thermal stability. The high-temperature stability (up to 250 degrees C) and high intrinsic proton conductivities of SPTES-50 qualify it to be a potential candidate for membranes in fuel cells. (C) 2007 The Electrochemical Society.