Nafion is a currently used as a proton exchange membrane in vanadium redox flow batteries (VRBs). It is an excellent proton-conducting and fully hydrated membrane, but it is costly. In order to reduce the cost of the membrane used in VRBs and to reduce vanadium permeability across the membrane, sulfonated poly(ether ether ketone) (S-PEEK) and sulfonated poly(1,4-phenylene ether ether sulfone) (S-PPEES) membranes were fabricated and studied as a function of the sulfonation reaction time. The increase in the degree of sulfonation from 46 to 86% induced increases in the water uptake, ion exchange capacity, proton conductivity, and vanadium permeability. The vanadium permeability coefficients of S-PEEK and S-PPEES membranes are in the range of 0-24.95 Â 10 -7 cm 2 min -1 , which are significantly lower than that of Nafion 117, which is 30.84 Â 10 -7 cm 2 min -1 . The proton conductivity of S-PEEK in our study is nine times higher than sulfonated poly(arylene ether ketone) in a previous work and more suitable for using in VRBs.