The reduction of U3O8 by hydrogen at 510 degrees C has been studied by thermogravimetry. After a first reduction to UO2, the sample is oxidised; then the analysis of the shape of the kinetic curves of a second reduction leads to the determination of the specific growth rate. according to a contracting-sphere model provided that the rate-limiting step occurs on the external surface of the particles. A mechanism of reduction is then proposed which accounts for the observed variations of the rate with the partial pressure in hydrogen.