CaCO3 was deposited by electrochemical precipitation in hard waters of various compositions. In situ Raman spectra of CaCO3 were recorded during the formation of the nuclei. The distinction between the three polymorphs (calcite, vaterite, and aragonite) combined with electrochemical and quartz crystal microbalance measurements allow the influence of the dissolved salt nature on the nucleation rate and the time required to a full coverage to be interpreted. It was shown that the scaling processes is modified by the influence of anions like sulfate, borate, or phosphate which induce formation of a hydrated layer of bicarbonate on the top of calcite crystals together with specific adsorption on the vaterite surface. In the case of sulfate and phosphate the crystallization of a pure calcium sulfate and pure calcium phosphate phases on calcite have been shown. Cations like magnesium or manganese substitute calcium in the calcite crystals. (C) 2003 The Electrochemical Society.