Accelerated degradation tests of selected membranes were carried out to determine their stability in the fully charged positive electrolyte solution of the vanadium redox battery. Each membrane was soaked in both 1.0 and 0.1 M V( V) solutions for extended periods of time and UV - visible spectroscopy was used to determine the rate of oxidation of the membrane by V( V) to produce V( IV) ions in solution. The membranes were then evaluated for any changes in their resistance, IEC, diffusivity and water transfer properties. FESEM was used to analyse the membranes for physical damage. Different trends were observed in the 1.0 and 0.1 M V( V) electrolytes. Of the membranes studied, Nafion 112E/ H+ showed the worst stability in the 0.1 M V( V) solution but one of the best stabilities in 1.0 M V( V). The dilute V( V) electrolyte appears to enter the pores of the membrane more readily as the membranes swell significantly in this solution. The 0.1 M V( V) solution therefore causes accelerated deterioration of the membrane performance as a result of physical destruction, chemical modi. cation or a combination of both. The effect is more pronounced in the membranes that have a higher degree of swelling in the vanadium electrolyte.