The stability of the protective oxide layer formed on Fe-12.5 pct Cr, Fe-18 pct Cr-10 pct Ni, and Fe-25 pct Cr-20 pct Ni was investigated in a flowing nonequilibrium H2-H2O-CH4-CO mixture at 950 °C. The samples were preoxidized in H2-H2O, and subsequently the carbon activity was raised in stepwise manner by adding methane. CO formation occurred on the sample surface by the catalytic reaction CH4 + H2O → CO + 3H2. The conversion of oxide to carbide commenced at the end of the sample wherePco has its highest andPH2O its lowest value. Oxidation, conversion, and catalytic reaction were monitored by gravimetric measurements and CO-determination during the experiments. Partial equilibria can be calculated for (ac)1 =K1Pch4/(Ph2)2, (ac)2 =K2(PcoPh2)ph{2o}and(Po2)1\2 = K3Ph2o/Ph2. The conversion of oxide to carbide started when the steady state ofac and(Po2)1/2 on the surface approached the value for the equilibrium of chromium oxide and chromium carbide. The results show that this steady state value is much nearer to the point(ac)2-(Po2)1/2 in the thermochemical stability diagram than to the point (ac)1 — (Po2)1/2- The oxide layers were stable even at very high values of (ac)1 which demonstrates that there is only very little interaction of CH4 with the oxide surface.