The change in zeta potential of CaCO3 crystal during reactive crystallization was studied. In the reactive crystallization with Ca(OH)(2) solution and CO2 gas, the zeta potential had a positive value, suggesting that Ca2+ was adsorbed on the surface of crystal. In the early stage of the reaction, the zeta potential of CaCO3 crystal was about +70 mV and decreased during the reaction. At the neutralization point, the zeta potential showed a minimum value of about +15 mV, after which it increased slowly. This dynamic change in the zeta potential during the reaction may reflect the change in the equilibrium between ions in the mother liquor and those on the crystal surface. To confirm this assumption, the zeta potential of the crystal sampled at the neutralization point was measured in mother liquors with various ionic compositions. The results showed that the change in zeta potential of the crystal reflects the change in the equilibrium at the crystal surface. The same measurement was conducted in the reaction with in the presence of polyacrylate acid (PAA) as polymeric additive. The zeta potential had a negative value, which suggests that PAA was adsorbed on the surface of crystal, and showed a dynamic change during the reaction. These results demonstrated that the change in zeta potential during the crystallization process reflects the change in the equilibrium between ions in the mother liquor and those on the crystal surface, information which will be useful for controlling crystallization processes.