Measurements of surface area reduction of TiO₂ anatase powder (of initially 100 m²* g⁻¹) were carried out for various partial pressures of water and oxygen at 823 K. The kinetic equation obtained for the experimental rates has the formulation r=k[_PH2O]1/[_PO2]1/β where alpha and β are equal to 2 and -12, respectively. A mechanistic model involving six consecutive elementary steps was developed, in which hydroxyl species play a dominant role. No geometrical assumption is required. The general expression of the deduced theoretical rates has the same form as previously given. The comparison between the experimental and the predicted rates points out that the rate-limiting step is the surface diffusion of hydroxyl species. This model can easily be used for any other compounds, for particle growth, and for porosity elimination.