When iron is oxidized at 500°C in oxygen at 10 Torr, blisters develop in the scale. This does not occur when the iron is oxidized at 500°C in oxygen at 700 Torr, nor when the iron contains carbon as a cementite precipitate. It appears that the iron oxide scale blisters if cation vacancies can accumulate at the oxide‐metal interface. The higher oxygen pressure inhibits scale blistering by promoting more rapid formation of the outer layer of , which lowers the rate of vacancy formation and accumulation. The presence of carbon in the substrate inhibits scale blistering by providing additional vacancy sinks, thereby largely preventing the accumulation of vacancies at the oxide‐metal interface.