In this study, the potential at which electrode reactions start and the effects of these reactions on membrane capacitive deionization (MCDI) were investigated. An adsorption experiment was conducted until the cell potential reached 1.2 V, applying constant current to an MCDI unit cell. While varying the current density (5.0-20.0 A/m(2)), the changes in cell potential, electrical conductivity and effluent pH were measured. The results of the experiment showed that the ion adsorption decreased significantly as the current density increased. Furthermore, the pH of the effluent rapidly decreased beyond a certain time in the adsorption process, which is judged to be the result of electrode reactions. When the electrode reactions started, the cell voltages were in the range of 0.62-0.90 V, which varied greatly depending on the current density. Using a simple model for the MCDI cell, the electrode potential was calculated from the cell potential and current density. Regardless of the current density applied, the electrode reaction was started at the electrode potential of 0.25 V. It is desirable to use the electrode potential instead of the cell potential to analyze the electrode reactions in MCDI operation. (C) 2014 Elsevier B.V. All rights reserved.