The surpassing of the solubility product of the anhydrous forms of calcium carbonate-calcite, aragonite, and vaterite-is not sufficient to induce spontaneous precipitation. The existence of a metastable zone, in the nucleation of the calcium carbonate, is still an experimental phenomenon. A thermodynamic demarcation of the metastable zone in the "CaCO3-CO2-H2O" system, where only a secondary nucleation can occur, has been delimited for the first time. Through experimental exploration of a large supersaturation field, results obtained by the bubbling method are treated with the use of thermodynamic data of different varieties of CaCO3. At temperatures ranging between 25 and 60degreesC a primary nucleation (spontaneous precipitation) occurs when the ionic activity product of the calco-carbonic solution surpasses the solubility product of (CaCO3H2O)-H-.. No spontaneous nucleation occurs when the ionic activity product stabilizes between the solubility products of calcite and monohydrated calcium carbonate, which means that the solution remains in a metastable state. The metastability can be broken by seeding with calcium carbonate crystals (aragonite in this case) and then the germination is a secondary one. A model for the prediction of the limit of the metastable zone, presented in this report, is in agreement with experimental results. (C) 2004 American Institute of Chemical Engineers.