The kinetic study of the transformation of cerium III hydroxycarbonate into ceria has beenachieved by means of thermogravimetry experiments and using a methodology based on the concepts of nucleation and growth. Two methods were used to determine the specific rates of nucleation and growth: a geometrical modeling which allowed to determine their variations against the partial pressures of gases involved in the transformation; an experimental method which gave the variations of the specific rate of growth. The comparison of the results obtained by these two methods allowed to validate the geometrical modeling and, thus, the values of the nucleation specific rate.