The anodic oxidation of an iron electrode in a neutral borate electrolyte is studied using a following ellipsometer. The oxide film is found to consist of two layers with different refractive indices: an outer layer of and a thicker inner layer of . A thin layer of is grown on the electrode in the active state, and the electrode passivates when a partial monolayer of is grown at the interface. Passive state anodic oxidation causes simultaneous growth of the two layers, the inner layer growing approximately 80% faster than the outer layer. We conclude that the layer inhibits anodic dissolution of the electrode, and that both the layer and the portion of the layer grown in the passive state act as the electrically limiting barrier across which the overpotential appears. The possibility that the two layers grow in a proportion dictated by the transport numbers of mobile defects (as has been proposed for the anodic oxide of tantalum) is discussed.