In this study, the oxidation of the V(II) by dissolved oxygen was examined quantitatively using UV-visible spectrophotometry. UV-visible spectrophotometry is an accurate method of determining the concentration of vanadium ions at both the negative and positive half-cell electrolytes. To apply Beer's law, the concentration should be diluted below 0.15 M to achieve a linear relationship between the absorbance and concentration. UV-visible spectrophotometry revealed that the concentration of V(II) in the negative half-cell electrolyte decreases continuously with cycling due to the rapid oxidation of the V(II) by dissolved oxygen. This decrease gives rise to an imbalance between the positive and negative half-cell electrolytes, which results in a significant capacity loss. (C) 2013 The Electrochemical Society. All rights reserved.