Mass transport phenomenon was investigated for the oxidation of Fe-Cr alloy in CH4-H2O at 1073 K. Grain boundary diffusion of elements was visualized around the oxide scale/alloy interfaces by secondary ion mass spectrometry (SIMS) imaging analysis. The distributions of elements on the oxide scale and alloy were analyzed in an area of 150 mum in diameter. The SIMS images show high concentration of Mn at the gain boundary. The depth profiles at the grain boundary from the images of Cr and Mn show high and constant concentration, suggesting fast diffusion of these elements. On the other hand, depth profiles at the bulk of the alloy show a specific distribution of each element from surface to inner of the scale: Mn-Fe-(Cr)-rich oxide, Cr2O3-rich, and SiO2-rich layers and alloy bulk with Al2O3 as internal oxides. The growth mechanism and scale formation kinetics are discussed. (C) 2004 Elsevier B.V. All rights reserved.