Measurements of surface area reduction of TiO₂ anatase powder (of initially 100 m²g^-1) are carried out at various partial pressures of water and oxygen and at 823 K. The kinetic equation for the experimental rates is obtained. We have developed a mechanistic model involving six consecutive elementary steps, in which hydroxyl species play a dominant role. No geometric assumption is required. The general expression of the deduced theoretical rates has the same form as the experimental rates. The comparison between the experimental rates with the predicted ones 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 porosity decrease.