It is suggested that, during the oxidation of stainless steels, matter is conserved at the oxide-metal interface by the creation of a dynamic balance between the chromium diffusion fluxes in the alloy and in the oxide. It is shown that the rate of oxidation is insensitive to alloy composition so that a necessary consequence is that the rate-controlling process is always diffusion through the oxide. In addition, the interfacial concentration of chromium remains invariant with time at a value higher than that in thermodynamic equilibrium with the oxide. Some of the predictions made with regard to the depth and kinetics of growth of chromium-depleted zones within the alloy have been checked experimentally inoxidation tests in CO2 at 1123° K on a 20Cr-25Ni stainless steel containing a dispersion of TiN particles. It is concluded that the matter-conservation hypothesis is valid for this material.