From the experimental rate for surface area reduction of anatase TiO2, expressed versus the partial pressure in H2O and the surface content of chloride ion, a kinetic model is proposed in which two parallel mechanisms are involved. Two ways of material transport appear to contribute to the surface area loss: in the vapor phase and at the surface of the oxide. The related rate-determining steps are attributed to the formation of the volatile compound Ti(OH)2Cl2 in the first case, and to the diffusion of hydroxyl ions in the second one. At high partial pressure of water, the rate is enhanced by surface interactions between water molecules and chlorine-containing species.