A series of sulfonated polysulfone (SPSF) proton exchange membranes with varied degree of sulfonation (DS) are prepared through controlling the molar ratio of sulfonated agent and polysulfone (PSF). An optimum DS for SPSF membranes is given by characterizing the physicochemical properties, ion selectivity and vanadium redox flow battery (VRFB) performance. SPSF membrane with DS = 62% (SPSF-62) demonstrates the best membrane performance, where the ion selectivity is 4.6 x 10(3) S min cm(-3) that is higher than that of Nafion 117 (4.0 x 10(3) S min cm(-3)). At current density of 50 and 100 mA cm(-2), coulombic efficiency (CE, 94.9% and 98.8%) and energy efficiency (EE, 89.2% and 86.2%) of SPSF-62 are much higher than that of Nafion 117 (CE, 91.3% and 96.5%; EE, 81.7% and 75.7%), respectively. In the meantime, the SPSF-62 membrane demonstrates a good durability over 100-time charging-discharging cycles. The present study of SPSF membranes proves a promising VRFB application as proton exchange membrane.