Summary: Electron-microscope observations of the oxidation products of olivine suggest that oxidation results in the formation ofplatelets of an ‘oxidized olivine’ phase, which has a superstructure of the olivine structure. Subsequently this intermediate phase breaks down by a cellular decomposition process to a eutectoidal intergrowth of magnetite and pyroxene. This intergrowth is responsible for the dendritic magnetite inclusions commonly observed in some olivines from Rhum.