A series of sulfonated poly(ether sulfone) copolymers (SPES-Xs) with varying degrees of sulfonation were prepared and investigated as ion-exchange membranes for vanadium redox flow battery (VRFB) applications. Sulfonated poly(thioether ether sulfone) copolymers (SPTES-Xs) were initially synthesized via polycondensation, and the SPES-Xs were then obtained by oxidation of the corresponding SPTES-Xs. The SPES-X membranes showed reduced vanadium-ion permeability, low area resistance, and, thereby, much superior selectivity compared with the parent SPTES-X membranes and a Nafion115 membrane. In single-cell VRFB performance tests, a SPES-50 membrane with an ion-exchange capacity of 1.80 meq/g exhibited a higher coulombic efficiency (>99%) and energy efficiency (76–89%) than the Nafion115 membrane over a wide range of current densities from 40 to 100 mA/cm² and a significantly larger capacity retention (>62%) during 200 charge-discharge cycles. The SPES-X materials, in which every benzene ring is deactivated by the presence of electron-withdrawing sulfone linkages, showed much better chemical stability during ex situ and in situ tests than the SPTES-X materials, which contain an electron-donating thioether linkage in their repeat unit.