The primary functions of membranes and separators in redox flow cells are to prevent the cross-mixing of the positive and negative half-cell electrolytes and to prevent short circuiting of the two half-cell electrodes while allowing the transfer of charge balancing ions during the passage of current. Another important property required of the membranes is their ability to prevent the preferential transfer of water from one half-cell to the other. The preferential transfer of water can be a problem in redox flow cells as one half-cell is flooded and diluted while the other becomes more concentrated, adversely affecting the overall operation of the cell. Most of the membranes evaluated for the vanadium redox flow battery have displayed good initial water transfer properties, but their performance deteriorated with long-term exposure to the vanadium solutions. In this study a range of low cost commercially available polyelectrolytes was selected for use in the modification of the cation selective Gore Select L-01854 membrane. Different types of polyelectrolytes were used to modify the membrane by soaking in a solution of the polyelectrolyte, the aim being to find an optimum treatment and to determine whether the nature of the polyelectrolyte influences the water transfer properties of the modified membrane. (C) 2003 Elsevier B.V. All rights reserved.