A conductive transparent electrode is used to observe and quantify the nucleation-growth process of scale deposition. The CaCO3 deposition is accelerated by increasing the interfacial pH induced by an electrochemical reaction on the electrode surface. Three measurements were simultaneously obtained with respect to time. The chronoelectrogravimetric and chronoamperometric curves showed that an increase of the supersaturation in the vicinity of the electrode increased the rate of scale deposition. This result was highlighted in the first instants of deposition when overlapping of crystals was not predominant. The third measurement was the in situ observation by a microscope through the transparent electrode. It allowed the count of crystals and the surface of the individualized crystals to be measured vs. time. It was shown that the increase of the local concentration of calcium and carbonate ions had no influence on the growth rate of both varieties of crystal present on the electrode surface, namely, vaterite and calcite. On the contrary, the count of the particles showed that the increase of the kinetic of scale deposition was only due to the enhancement of the nucleation rate. Thus, for the lowest investigated supersaturation, the number of nuclei increased linearly with time, whereas the highest supersaturation promoted a faster nucleation rate and a higher proportion of calcite. (C) 2003 The Electrochemical Society.