Oxidation and reduction are utilized in the refining process of iron and steel. Therefore, the most important thermodynamic property is the activity of FetO in slag and especially in refractories, which contact with liquid iron and steel. It is well known that refractories easily form solid solutions with FetO, so that molten iron and steel would react with FetO in the refractories, non‐metallic inclusions and slag coexisting in the iron and steel production process. In the present work, the reaction of molten steel with the solid oxide phase will be discussed based on the activity of the constituents in MnO‐FetO and FetO.Cr2O3‐FetO solid solutions by way of examples of FetO‐containing solid solution.