In the present studies, Daramic, which consists of ultrahighmolecular polyethylene, amorphous silica and mineral oil, was crosslinked with divinylbenzene (DVB) as a crosslinking agent and further sulfonated to obtain a cation exchange membrane for the vanadium redox cell. It was shown that the average pore size of Daramic was reduced from 0.1 to 0.02 mu m after crosslinking [1]. The preparation and characterisation of the modified ion exchange membrane from the composite membrane with subsequent sulfonation are described. The membrane modification process results in a dramatic reduction in solvent transfer across the membrane when used in the vanadium redox cell. The ion exchange capacity (IEC) of the sulfonated composite membrane was also evaluated. The sulfonation reaction was found to be able to incorporate cation exchange groups into the composite membrane. Water transport measurements across the sulfonated composite membrane showed promising results compared with those across the composite membrane. A detailed FESEM and C-13 NMR analysis of the composite membrane and the sulfonated composite membrane have been carried out.