Oxide scales formed on an Fe-Cr alloy in fuel gas atmospheres (H-2-H2O and CH4-H2O) were investigated by secondary ion mass spectrometry (SIMS). From surface to oxide scale/alloy interfaces, Mn-Fe-(Cr) spinel, Cr2O3, SiO2, and Al2O3 inner oxides were identified by SEM and SIMS depth profiles. The elemental distribution analysis suggests that the grain boundary diffusion in the alloy plays an important role on growth of oxide scales. The growth of oxide scale followed the parabolic relationship between scale thickness and annealing duration time. The estimated growth rates of oxide scales are in an order of 10(-14) cm(2) s(-1) at 1073 K. Oxygen diffusion through the oxide scales were evaluated by 180 diffusion with SIMS analysis. The diffusion coefficients of O-18 in the oxide scales were 3.1-5.4 x 10(-12) cm(2) s(-1) at 1073 K. which were larger than the obtained growth rates of the oxide scales. (C) 2004 Elsevier B.V. All rights reserved.