The water uptake, ionic conductivity, vanadium (VO2+) permeability and stability of polysulfone (PSF) based AEMs in alkaline media and in strongly oxidizing solutions were assessed. The highest ion conductivity was obtained with PSF-trimethylammonium (TMA)+. PSF-TMA+ also had better alkaline stability in comparison to PSF-AEM with different bases. PSF-TMA+ was demonstrated to show fuel cell performance. PSF-TMA+ demonstrated a 40-fold reduction in vanadium (VO2+) permeability when compared to Nafion® membrane. Comprehensive 2D NMR studies verified that PSF-TMA+ remained chemically stable even after exposure to a 1.5 M vanadium(V) solution for 90 days. Excellent energy efficiencies (85%) were attained and sustained over several charge–discharge cycles for a vanadium redox flow battery prepared using the PSF-TMA+ separator.