Precipitated calcium carbonate particles were prepared by passing CO2/N-2 mixed gases into CaCl2 solution. The nucleation, crystallization and transformation of vaterite at controlled pH were investigated in this paper. The control of pH was conducted by the addition of ammonia. The results showed that addition of ammonia induced a local high supersaturation and continuous nuclei formation, which is in favor of the growth and agglomeration of vaterite nuclei while inhibiting the transformation of vaterite to calcite. The nearly pure vaterite is ready to form at the control of pH. (c) 2005 Elsevier B.V. All rights reserved.