An investigation has been undertaken into the oxidation behaviour of manganese-containing Fe-28% Cr alloys in oxygen at 800" and 1OOO"C. The presence of the tertiary element has a detrimental effect on the oxidation resistance, resulting in enhanced scale-growth rates during isothermal exposure and increased incidences of scale failure at temperature. This is largely due to relatively rapid rates of diffusion of manganese across the Cr203 scale and formation of MnCrzOd spinel on its outer surface. The scale on Fe-28% (31% Mn consists of a layer of Cr203, containing a small concentration of manganese, with an outer layer of the spinel oxide. During the early stages, an inner layer of the spinel also develops, but, eventually, this almost completely disappears as the manganese diffuses into the outer scale. A similar scale forms on Fe-26% 0 3 % Mn, but the higher manganese concentration enables a significant amount of this element to be retained in the inner regions. The overall growth rate of the scale is significantly faster than on Fe-28% Cr or the 1% Mn-containing alloy.