The charge efficiency of electrochemical capacitive deionization (CDI) of salty-brackish water in symmetric cells comprising activated highly porous carbon electrodes was investigated as a function of the applied potential during charging (deionization) and discharging. The charge efficiency of water CDI in such cells is highly affected by the fact that the potential applied drives always simultaneously adsorption of counterions and desorption of co-ions. A previous stage of this work concentrated in the study of single activated carbon electrodes in processes. In this study, we demonstrated that it is possible to estimate the charge efficiency of symmetric CDI cells from measurements of single electrodes. Guidelines for improving the charge efficiency in water desalination by CDI processes are outlined herein. For instance, it was demonstrated based on experimental findings that it is possible to maximize the charge efficiency by applying discharge potentials (upon regeneration) higher than zero. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3193709] All rights reserved.