In order to observe in real time and in situ the electrocrystallization of calcium carbonate, a new method using an assessment of the oxygen reduction reaction has been developed. The experimental assembly is composed of an electrochemical cell and an optical-video set-up. The specific features of the cell are: availability of an electrode polarization control, enabling the use of any kind of cathodic surface and a transparent anodic window. Connected to a video set-up, an optical assembly monitors and records the surface area of the working electrode during the experiment. An analysis system quantifies the number and the mean characteristics of the crystals. The results obtained with synthetic and natural waters are set out. Fresh synthetic water (10 degrees F) presented low progressive nucleation and strong growth of the small number of crystals; vaterite was predominant. The nucleation stage was instantaneous and sizeable in waters with a degree of hardness of 30 degrees F and 50 degrees F. At 50 degrees F, calcite was the main crystalline form confirming that there is a strong correlation between nucleation and crystalline forms in synthetic water.