Crystallization processes were investigated by in situ microscopic observation through a transparent electrode. In calcareous scale deposition, the technique led the nucleation-growth process of each crystalline form to be studied vs. time. During the first moments of growth, where overlapping did not occur, the number of particles and the area of each individual crystal increased linearly with time (t) in the present experimental conditions. Consequently, the total covered surface was proportional to t(2) showing a progressive nucleation.