The reduction of U3O8 by hydrogen has been studied by thermogravimetry in the range 510-600°C and 1-40 kPa hydrogen partial pressure. The kinetic curves have been interpreted by a new model involving steps of nucleation and growth in which the rate limiting step of growth is located at the external interface. This allowed us to determine the variations in the areic nucleation frequency and the areic reactivity of growth as a function of hydrogen pressure and temperature. Mechanisms have been proposed which account for the variations in the areic nucleation frequency and reactivity of growth as a function of the partial pressure of hydrogen and the temperature of reduction. The influence of the hydration of UO3 prior to its reduction by hydrogen on the kinetics could also be explained by the same model. It was found to lower the areic nucleation frequency without changing the areic growth reactivity.