In order to study the effects on oxidation behavior of changing the composition of oxide scales without changing the composition of the underlying alloys, thin films, which contained various amounts of Y{sub 2}O{sub 3}, Cr{sub 2}O{sub 3}, or Al{sub 2}O{sub 3} have been deposited on Fe-25%Cr and Fe-25%Cr-5%Al prior to oxidation. There were major differences between the (Fe-25%Cr)-(Cr{sub 2}O{sub 3}-Y{sub 2}O{sub 3}) and (Fe-25%Cr-5%Al)-(Al{sub 2}O{sub 3}-Y{sub 2}O{sub 3}) systems. A film of Cr{sub 2}O{sub 3} on Fe-25%Cr reduced the oxidation rate and Cr{sub 2}O{sub 3}-Y{sub 2}O{sub 3} films reduced it still further. A film of Al{sub 2}O{sub 3} on Fe-25%Cr-5%Al reduced the rate considerably, but Al{sub 2}O{sub 3}-Y{sub 2}O{sub 3} films exhibited rates that were greater than those for Al{sub 2}O{sub 3} alone. A film, which consisted entirely of Y{sub 2}O{sub 3}, greatly decreased the oxidation rate of Fe-25%Cr, but increased that of Fe-25%Cr-5%Al. The changes in oxidation rate produced by Y{sub 2}O{sub 3} in the (Fe-25%Cr)-(Cr{sub 2}O{sub 3}-Y{sub 2}O{sub 3}) system were much greater than those produced in the (Fe-25%Cr-5%Al)-(Al{sub 2}O{sub 3}-Y{sub 2}O{sub 3}) system. As far as oxidation rate is concerned, the optimum amount of Y{sub 2}O{sub 3} in the (Fe-25%Cr)-(Cr{sub 2}O{sub 3}-Y{sub 2}O{sub 3}) system was found to be greater than or equal to 40% and less than 100%; in the Fe-25%Cr-5%Al-(Al{sub 2}O{sub 3}-Y{sub 2}O{sub 3}) system it was approximately 10%. In thermal-cycling tests, the results for the (Fe-25%Cr)-(Cr{sub 2}O{sub 3}-Y{sub 2}O{sub 3}) system followed the same pattern as that for the isothermal tests; a film of Y{sub 2}O{sub 3} reduced the oxidation rate and a film of Cr{sub 2}O{sub 3}-33%Y{sub 2}O{sub 3} reduced it still further.