The decomposition of calcium carbonate has been studied by isothermal and isobaric thermogravimetry at 700 °C, under various pressures of carbon dioxide. The kinetic curves, which have a sigmoidal shape, have been interpreted with a model involving nucleation and growth processes based on the Mampel's model. It was supposed that nucleation occurs at the surface of all the particles and that growth proceeds isotropically inward in each of them. This model allowed to characterise the chemical reactivity of several CaCO3 powders by means of the determination of both their specific nucleation frequency and their specific growth rate.