The oxidation of a pure Fe‐18.0% Cr alloy in oxygen at 1 atm pressure in the temperature range 800°–1200°C has been studied using thermogravimetry, metallography, and electron probe microanalysis. Chromium oxide, containing minor amounts of dissolved iron, is the only oxide formed at 800° and 1000°C. This oxide is also formed initially at 1200°C, but its subsequent mechanical failure permits rapid oxidation of the underlying chromium‐depleted alloy. Three types of scale formation are found in this second oxidation stage on different regions of the specimen surface. Each of these types of scale contains alternate layers of chromium‐rich and iron‐rich oxides. Two of the scale types are eventually healed by formation of a basal ‐rich layer; the third, occurring at the edges of the specimens, grows unimpeded. The mechanism of formation of these stratified scales and, where relevant, their subsequent healing, is explained by a theory involving the variation of the mobilities of Fe3+, Fe2+, and Cr3+ ions with composition in the spinel, .