The influence of the pH at the electrode/solution interface on CaCO3 precipitation by means of electrochemistry was studied both theoretically and experimentally in pure calcocarbonic aqueous solutions. This interfacial pH was measured at the electrode by using a pH sensor-grid assembly in a submerged impinging cell. A model which takes into account the electrochemical oxygen reduction and the chemical dissociation of hydrogen carbonate and carbonic acid is proposed to calculate the pH at the electrode/solution interface. It can reproduce the data using values of adjustable parameters obtained from experiments. Hence, the local concentrations of OH-, HCO3-, CO32-, Ca2+, and the supersaturation coefficient, beta, were determined at the electrode. These data were used to interpret CaCO3 precipitation on a gold electrode. (C) 2003 The Electrochemical Society. All rights reserved.