The effects of seven carboxylic acids, i.e., acrylic acid, maleic acid, tartaric acid, malic acid, succinic acid, and citric acid, on CaCO3 crystallization were studied using the unseeded pH-drift method along with a light-scattering technique. Experiments were started by mixing solutions of CaCl2 and NaHCO3 in the presence or absence of additives. The crystallization was studied by recording the decrease in pH resulting from the reaction Ca2+ + HCO3- --> CaCO3 + H+. A given amount of carboxylic acid was added to the solution of CaCl2 or NaHCO3 before mixing the reactants. The pH profiles obtained in the case of the CaCl2 solution containing an additive were similar to those for the NaHCO3 solution containing one, and when an additive was added after the onset of crystallization, the growth of CaCO3 immediately stopped. The light-scattering observations, in all cases, indicated that CaCO3 nucleation occurred at 10-20 s after mixing of the reactants. The results indicated that the nucleation of CaCO3 was not influenced by the presence of carboxylic acids, but CaCO3 crystal growth was reduced by their adsorption to the surface of the CaCO3 crystals. These phenomena were explained by assuming a stronger affinity of the carboxylic acids for CaCO3 particles than for the free Ca2+ ions in solution. The crystallization of CaCO3 in the presence of additives was divided into three stages: nucleation, growth incubation, and growth periods.