A carbonation route for synthesizing hydrophobic CaCO3 nanoparticles was described via an organic substrate inducing process. In this study, the synthesized octadecyl dihydrogen phosphate (C18H37OPO3H2) associated with Ca2+ by means of molecular recognition and induced the nucleation and growth of CaCO3. The CaCO3 nanoparticles were prepared by carbonation of a mixture of Ca(OH)(2) and C18H37OPO3H2 with CO2/N-2 gas mixture. The particle sizes and morphology of the resulting samples were characterized by transmission electron microscopy (TEM). The results indicated that the spindle-like CaCO3 particles with a mean diameter less than 100 nm were prepared in our study. Fourier transform infrared spectroscopy (FT-IR) analysis revealed that the organic substrate associated with CaCO3 based on the appearance of P-O symmetric stretching vibrations at 1077 and 578 cm(-1). The CaCO3 nucleated in the presence of organic substrate exhibited different endothermic peak compared to one nucleated in the absence of organic substrate in different temperature and thermal gravimetric analysis (DTA-TG). The X-ray diffraction (XRD) result revealed that the particles obtained in this study were calcite. In order to observe the effect of the concentration of organic substrate on the morphology, size and hydrophobic property of CaCO3, the concentration of organic substrate was increased from 0.25 (m/v) to 1.00 (m/v). (c) 2006 Elsevier B.V. All rights reserved.