The current paper describes the influence of trace amounts of Cu2+ ions on the precipitation of calcite in an aqueous and supersaturated CaCO3/NaCl system and in natural water of moderate hardness. Furthermore the influence of Zn2+ ions on CaCO3 precipitation in the CaCO3/NaCl system was examined and the results were compared. The resulting reduction of the Ca2+ Concentration was recorded as a function of time by measuring the electric conductivity and the pH value. Because of the metal species present in the solution, two competing processes take place. Neutral CuCO30(aq) and ZnCO30(aq) complexes are adsorbed onto the active growth sites of the CaCO3 surface and thus, further crystal growth is inhibited. As a result of nucleation catalysis, Cu(OH)(2) particles cause the accelerated formation of CaCO3 nuclear crystals, which counteracts the inhibition process. At Cu2+ concentrations > 1.40 mg/L, Cu(OH)(2) is transformed into stable and crystalline malachite, at which no catalysed nucleation can take place. This results in a significant increase of the induction period and inhibition, while the efficiency attained is about 90% for Cu2+ contents >2.00 mg/L For an addition of Zn2+ ions, ZnCO3 is precipitated and acts as strong accelerator of heterogeneous nucleation of CaCO3, resulting in an enhanced growth rate which dominantly counteracts the inhibition effect. The results may be explained, that copper ions are more effective for scale prevention than zinc ions.