Hydrophobic vaterite nanoparticles were prepared via crystallization of CaCO3 with oleic acid in methanol by mimicking the process of biomineralization. The molar ratio of oleic acid to calcium ion was varied from 0.1 to 0.5. By changing the concentration of the oleic acid, CaCO3 particles with different shapes and polymorphism were obtained. High concentration of the oleic acid gave stable vaterite crystals, the polymorph of which did not change when the composite was kept in water for more than one week. Fourier transform infrared spectroscopy (FT-IR) and TGA analysis of the obtained product indicated that the oleic acid was bound to the crystalline CaCO3. The contact angle of the modified vaterite reached 122 degrees. We have succeeded in crystallization of hydrophobic CaCO3 nanoparticles in situ. (c) 2006 Elsevier B.V. All rights reserved.