Nafion® is a currently used Proton Exchange Membrane (PEM) in Vanadium Redox Flow Batteries (VRB). It is an excellent proton conducting and fully hydrated membrane, but it has a very high cost. In order to reduce the cost of the membrane used in VRB and to reduce the vanadium permeability across the membrane during the operation, sulfonated poly (ether ether ketone) (PEEK), and poly(1,4-phenylene ether ether sulfone) (PPEES) membranes were fabricated and studied as a function of the sulfonation reaction time. The increase in the degree of sulfonation (DS) from 46 to 86% induces the increase in the water uptake, the ion exchange capacity (IEC), the proton conductivity, and the vanadium permeability. The vanadium permeability coefficients of S-PEEK and S-PPEES membranes are in the range of 0 to 24.95 x 10⁻⁷ cm² min⁻¹, which are significantly lower than that of Nafion® 117, which is 30.84x10⁻⁷ cm² min⁻¹.