The systematic study of hydration kinetics under controlled temperature and water vapour pressure, for an initial sample mass m o = 100 mg, has established three areas in the stable domain of the hydrated phase, (i) A region designated R I, at high pressure and high temperature where deviation from the hydrate-anhydrous equilibrium, and volumic phenom- ena are determining, (ii) A second region R II, at low pressure and low temperature near the hydrate-saturated solution equilibrium, where the deviation from this equilibrium, and surface phenomena become in their turn determining, (iii) In the intermediate region R III it seems that the hydration kinetics are regulated by the interface progression. In this paper, we observed that the initial mass of the sample influences the kinetic phenomena. (1) If the mass is great ( m o = 1000 mg), the regions R I and R III disappear in favour of a region denoted R' I which occupies the greater part of the stability domain, corresponding to an intercrystalline and not to an intracrystalline volumic diffusion, as is the case in the region Ri defined above. (2) The variation of the initial mass of the sample is shown by the displacement of the border of the regions R II, R III and R' I. A change in the kinetic mechanisms (diffusion in the reaction bed for high mass, surface adsorption and budding for low mass, and interface progression for mass near m o = 300 mg) between these three regions has been observed with decrease of the initial mass of the sample at constant temperature and pressure. It is apparent from this analysis that the use in a system of storage energy of a finely divided quantity of mass leads to a better transfer of mass and therefore of heat, and leads to a fast transformation.