A series of perfluorosulfonic membranes is screened for application in vanadium redox flow batteries (VRFB): membranes of constant thickness 50 mu m with different ion-exchange capacities ranging from 0.56 to 1.15 mol eq.g(-1). Diffusion flux of each vanadium ion occurring in VRFB electrolytes through each examined membrane is measured by UV/Vis spectroscopy. Permeation of V2+ ions contributes most to the self-discharge losses and the mechanism of vanadium ions permeation is discussed for all oxidation states. The membranes are characterized in the single-cell by electrochemical impedance spectroscopy, load curve measurements with linearly increasing current and charge-discharge cycles at various current densities ranging from 50 to 200 mA cm(-2). Generally, at lower current densities the permeation of vanadium ions decreases the battery efficiency and thus lower ion-exchange capacity membranes are more suitable. Concurrently, at higher current densities the battery efficiency is decreased by the membrane resistance and thus higher ion-exchange capacity membranes are optimal. However, membrane ion-exchange capacity is not the only characteristic that affects the VRFB performance. Small-angle X-ray scattering of membranes revealed the effect of polymer molecular architecture on the size of hydrophilic domains which affects the membrane transport properties.