In this article the effects of several operating variables, including supersaturation, temperature, and the presence of additive and seeds on the induction period of CaCO3 were studied experimentally and theoretically. In experimental, the induction period was measured by a conductivity method so that the induction period was well identified. The results show that the induction period increases with decreasing supersaturation and temperature. The presence of Mg2+ in solution prolongs the induction period and the addition of seeds reduces the induction period. In theoretical, a cluster coagulation model, which brings together the current nucleation models and the theories describing the behavior of colloidal suspensions, was applied to estimate the induction period under various operating variables. A comparison between the two results shows that the present model is a suitable one to interpret the effects of various operating variables on the induction period of aqueous CaCl2-Na2CO3 solution. (C) 2003 Elsevier Ltd. All rights reserved.