The nature of carbon steel surfaces in 0.01M borate solutions (pH 10.6) have been characterized using a range of electrochemical techniques and ex situ analyses such as Raman and Auger spectroscopy. Their subsequent behaviour on exposure to 10⁻³ M H₂O₂-containing solutions has also been studied. The anodically oxidized carbon steel surfaces have been characterized according to three regions: (I) the potential range <−0.5V (vs SCE), when the surface is active and covered by Fe^II/Fe^III oxide/hydroxide; (II) the potential range −0.5V to ∼0.0V when the surface is passivated by an outer layer of Fe^III oxide/hydroxide over the inner layer of Fe^II/Fe^III oxide/hydroxide; and (III) potentials >0V when further growth of the underlying layer appears to lead to minor film breakdown/restructuring. The addition of H₂O₂ to films grown in the passive region or above (II and III) leads initially to a degradation of the outer layer allowing increased growth of the inner layer. Subsequently, the outer passivating layer is repaired and passivity re-established. These changes appear to be confirmed by Raman spectroscopy.