Water transfer behaviour across commercially available cation exchange membranes in the vanadium redox battery (VRB) was investigated. Evaluations showed that the direction of preferential water transfer was dependent on the state of charge (SOC) of the vanadium electrolytes employed. When the vanadium electrolytes are at an initial SOC of 100 and 50% the direction of water transfer is toward the positive half-cell. As the electrolyte discharges from 50 to 0% SOC the direction of water transfer reverses toward the negative half-cell. It appears that the most significant level of water transfer occurs as the cell goes into over-discharge where the V(III) and V(IV) electrolytes become fully mixed below the 0% SOC state. The membranes evaluated displayed comparable water transfer properties. (C) 2003 Elsevier B.V. All rights reserved.