Due to its potential for fuel cell applications, the synthesis of sulfonated poly(phenylene sulfone) with an ion-exchange capacity (IEC) of 2.78 mequiv g À1 (sPPS-360) is optimized and the effect of increased molecular weight on mechanical properties studied.The synthesis comprises the step-growth polymerization of sulfonated difluorodiphenylsulfone (sDFDPS) with thiobisbenzenethiol (TBBT), followed by an oxidation (via hydrogen peroxide). In the past, this procedure was marked by very low reproducibility and often limited achievable molecular weight. Sodium sulfate-a byproduct in the preparation of sDFDPS-is found to be retained throughout the monomer's established purification procedure via recrystallization from alcohol/water mixtures. The contamination is quantified by atomic absorption spectroscopy (AAS) and combustion analysis. At corrected monomer ratios, the preparation of sPPS-360 is highly reproducible and a subsequent optimization of synthetic parameters yields a significant increase in number average molecular weight (typical M n 15-30 kDa up to 154 kDa). Improved viscoelastic properties of optimized sPPS-360s are evident in tensile tests at 30 %RH, better membrane-forming properties and reduced water uptake (when submerged in water).