The microstructure of the oxides formed on F91 steel upon breakaway oxidation in air and CO2 at 700 °C was characterized and compared. It was observed that the Cr-rich corundum layer became humped and disrupted upon breakaway oxidation in air in the first stage. In contrast, spinel was found to form in CO2 without any humping of the Cr-rich layer upon breakaway oxidation. Whether or not the humping and disrupting of the Cr-rich layer were present was related to the form of oxide. Specifically, corundum oxide was facile in air, whereas spinel oxide was facile in CO2.