A study of the cathodic reduction of on iron has been made with the object of using the technique for the accurate determination of both the and formed during the oxidation of iron. The films were examined after oxidation and after various stages of cathodic reduction. Examination was made by weight change, electron diffraction, x‐ray diffraction, and chemical analysis of the films. Some evidence for the existence of a thin layer of between the layers of and was found. A standard pattern for was obtained.