Chlorosulfonated homogeneous polyethylene (PE) dense film (PE-X) and asymmetric membrane (MH-X) were tested as separators for the all-vanadium redox flow battery. The membranes are prepared by the vapour phase chlorosulfonation of the PE film. The measured lowest resistivites equilibrated in 2 M KCl aqueous solution were 0.27 Omega cm(2) and 0.96 Omega cm(2), respectively, for PE-X (with thickness 20 mu m) and for MH-X (with PE-layer 20 mu m). The area resistivities of the membranes as separators in the all-vanadium redox flow battery were obtained. At a charge-discharge current density 633 A/m(2), these values were 3.09 Omega cm(2) and 3.46 Omega cm(2), respectively, for charge and discharge PE-X, and were 3.26 Omega cm(2) and 8.30 Omega cm(2), respectively, for charge and discharge MH-X. The highest permeabilities of the four types of vanadium ions had PE-X, followed by Nafion 117, CMV, and MH-X. For the purpose to improve the durability of the membrane, the chlorosulfonated homogeneous membrane was cross-linked by accelerated electron radiation. The life of the cross-linked membrane which is determined by the oxidation degradation in acidic vanadium solution, was longer than it for non-cross-linked membrane and CMV.