Abstract The working principle of the electrochemical softening process was studied at the laboratory scale in order to get a better understanding and to optimize the construction and the efficiency of industrial electrolysers. For this purpose pure calco carbonic synthetic waters with well defined hardness or distribution water from Paris were used. By using a local pH sensor, it was shown that the increase of the pH in the very vicinity of the cathode induces the precipitation of the calcium carbonate on the plate, in a first step under the effect of the oxygen reduction, and after that by water reduction associated with hydrogen evolution. The local pH can reach values greater than 10. The morphology and the crystal form (calcite, vaterite and/or aragonite) of the deposits were identified by scanning or transmission electron microscopy and by X ray diffraction. It was shown that, at the beginning of the treatment, vaterite and calcite crystals form a compact layer. In galvanostatic conditions, the decrease of the active area by deposition of the insulating scale leads to an increase of the local current density and then, to the transition towards the electrolytic water reduction regime. Adendritic growth of the calcium carbonate forming a porous layer through which hydrogen diffuses easily is observed. An investigation carried out on a model scale electrolyser showed the influence of various operating parameters such as current intensity, time of treatment etc. on the efficiency of the device. This electrochemical process is also able to eliminate partially various other species like magnesium.