Smoluchowski's coagulation theory is adopted to estimate the induction period for the formation of the critical nuclei of CaCO3 in a supersaturated CaCl2-Na2CO3 aqueous solution. The effects of the presence of impurity and crystal seeds, degree of supersaturation and operating temperature on the induction period are discussed. The association coefficient between two clusters is found to depend on both the degree of supersaturation and the temperature. The estimated activation energy for the association between two clusters is about 200 J/mol, which implies that the association between two clusters is of physical nature. It is found that the increase in the induction period with the presence of Mg2+ may be caused by an increase in the solid-liquid interfacial energy, and a decrease in the association coefficient. Due to van der Waals attraction, the addition of crystal seeds increases the concentration of clusters near seed surface, and, thus, yields a shorter induction period. (C) 2005 American Institute of Chemical Engineers.