In this study, the concentration of vanadium ions in a carbon felt electrode was examined quantitatively using UV-visible spectrophotometry with respect to the current density and flow rate to examine the effect of the concentration overpotential on the performance of a vanadium redox flow battery (VRB). Experimental results revealed that the concentration of vanadium ions in the positive electrode is different from that observed in the positive electrolyte tank, and the deviation increases with increasing current density and decreasing flow rate. The concentration of V(IV) is higher in the positive electrode compared to the positive electrolyte tank after the discharging process, which leads to a premature cut-off in the discharge of the VRB. Therefore, this result suggests that it is important to improve electrolyte transport through the electrode for better performance of VRBs, and the measurement of the concentration of vanadium ions in the electrode could be useful for studying the concentration overpotential. (C) 2014 The Electrochemical Society. All rights reserved.