The lattice and grain boundary diffusion coefficients of18O(at 0.1 atm) were determined at 900°C in bulk Cr2O3, and in Cr2O3scales developed on a Ni70Cr30alloy. The diffusion profiles were established by SIMS and analyzed considering two domains, the first one relative to apparent diffusion and the second to grain boundary diffusion. With such an analysis and taking a f value = 0.0006, modified from a ridge model, the oxygen lattice diffusion coefficients determined in Cr2O3scales are then in very good agreement with those in bulk Cr2O3. With some assumptions, our diffusion data lead to a parabolic oxidation constant equal to the experimental one. The scale growth is ensured by counter-current diffusion of oxygen and chromium, mainly by grain boundary diffusion.