The textural changes of two zirconium dioxide powders, one containing only tetragonal crystallites and the other a mixture of monoclinic and tetragonal crystallites, were investigated. The crystallite size increase was followed by means of X-ray diffraction line broadening for powders annealed in a controlled gaseous atmosphere at 600 and 770°C, in order to establish the variations of its experimental rate versus partial pressures of water and oxygen. An accelerating influence of water vapour was observed for both powders. A model, based on a catalytic effect of water vapour, and involving six elementary steps, was proposed in order to calculate theoretical rates within the approximation of the rate-limiting step. The comparison between the experimental and theoretical rate laws attests to the validity of the model and defines the most probable rate-limiting step: tetragonal crystallite growth proceeds at a rate controlled either by adsorption (or desorption) of water molecules or diffusion of hydroxyl groups: monoclinic crystallite growth is controlled by the diffusion of zirconium ions.