A carbonation route for the synthesis of nanosized calcium carbonate (aragonite) was studied. In the process, polyacrylarnide was used as an organic substrate to induce the nucleation and growth of calcium carbonate. The calcium carbonate particles were produced by means of carbonation of the mixture of calcium hydroxide and polyacrylamide by bubbling CO2/N-2 gas mixture. The operating parameters such as the concentration of organic substrate and temperature were varied to study their influences on the polymorph and crystal sizes of calcium carbonate particles. The morphology of the calcium carbonate particles was characterized with transmission electron micrograph (TEM). The synthesized calcium carbonate particles in the presence of organic substrate are the mixture of aragonite with needle shape and calcite with cubic shape. Fourier transform infrared spectroscopy (FTIR) analysis reveals the presence of aragonite and calcite. The polymorphs and their crystal sizes were characterized with X-ray diffraction (XRD). The calcium carbonate nucleated in the organic substrate exhibited different endothermic peak in differential thermal gravity (DTG) results compared to one coexisting with the organic substrate. (c) 2006 Elsevier B.V. All rights reserved.