Capacitive deionization is a method in which water is desalinated by the application of a potential between two porous electrodes. Previous reports generally consider that the applied potential is usually divided in half for both electrodes and that the active mass of the electrodes plays an important role on the desalination performance obtained from a certain CDI system. In this work, a potentiostatic method for the determination of the electrochemical stable potential window (ESPW) was used, and thus an optimised mass ratio for the electrode pairings, of a cell made with activated carbon electrodes and a concentrated NaCl aqueous electrolyte, was found. The obtained results demonstrate that the ESPW in which the electrodes would not suffer from Faradaic degradation is 0.4 V for the positive electrode and −0.6 V for the negative electrode, leading to a mass balance ratio of m+/m- = 2.22. These parameters were then applied to a CDI cell, in which an improvement of 65% of the salt adsorption capacity and of 58% for charge efficiency was obtained when the system was operated with the mass balanced conditions, compared to the equivalent mass case.