The reduction of UO3 by hydrogen has been studied by thermogravimetry in the range 510-600°C and 1-40 kPa. The kinetic curves have been interpreted by a new model involving the steps of nucleation and growth. This allowed to determine the variations of the frequency of nucleation and of the specific rate of growth as a function of hydrogen pressure and temperature. The influence on the kinetics of the hydration of UO3 prior to its reduction by hydrogen could also be explained due to the same model.