During oxidation of a Laves-phase strengthened ferritic steel at 800 degrees C in Ar/H-2/H2O a precipitate-depleted zone occurs as a result of subscale chromium depletion. In parallel, an enrichment of Laves-phase is found in the immediate vicinity of the scale/alloy interface. Modeling the phase equilibria and diffusion processes for an Fe-Cr-Nb model alloy using Thermo-Calc and DICTRA revealed that this combined enrichment/depletion process can be explained by the influence of alloy chromium concentration on the Nb activity. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3590258] All rights reserved.