Iron oxides are reduced in the following sequence:
     O  →    FeO     →   FeO
Each transition from one oxide to the next is due to two simultaneous high-temperature reduction reactions by carbon monoxide CO or dihydrogen H:
These temperatures differ from those predicted by the Ellingham diagram. In reality, there is a coupling between carbon monoxide reduction and dihydrogen, so that these reactions work together, with hydrogen significantly improving the efficiency of CO reduction.