Oxidation of high purity iron in CO2 and CO2 + COmixtures at different total pressures (0.1–1 atm.) has been studied at 1000–1200°C. While paper I of this study emphasized studies of the relation between scale morphology and reaction kinetics, this paper focuses on the reaction mechanism during the initial oxidation involving growth of wustite films and scales. The reaction behavior is analyzed in terms of coupled kinetics comprising a surface reaction and diffusional transport through the scale. A classical model derived by C. Wagner, based on the assumption that the properties and defect concentrations in the scale surface are exactly the same as in bulk wustite equilibrated in CO2 +CO mixtures, does not provide a satisfactory description of the reaction kinetics and the gas-pressure dependence of the surface reaction. As an alternative model, it is suggested that the gasous molecules/species interacts with the surface to form surface complexes, and that surface complexes withCO2serve as preferred reaction sites for the surface reaction.