Calcium carbonate was precipitated from carbonically pure solutions by the electrochemical method onto various metallic substrates. A special electrochemical cell was designed to allow in situ micro-Raman spectroscopy to be carried out during the scaling process with a controlled solution flux on the electrode surface. By this method it was possible to identify the polymorphic variety, among the three possible, of individual precipitated calcium carbonate crystals, and to study the effects of various parameters on the crystal structure of the nuclei. It was confirmed that a strong correlation exists between the rate of nucleation and the crystal polymorph. The nucleation rate depends on the nature of the substrate. It is shown that the presence of a passivating layer on non-noble metals reduces the density of nucleation sites and thus promotes the formation of vaterite nuclei. Further growth of aragonite crystals involves a total phase transformation from vaterite nuclei into aragonite. (C) 2003 The Electrochemical Society.