Samples of the iron oxides Fe0.94O, Fe3O4, Fe2O3, and Fe2SiO4 were prepared by high temperature equilibration in controlled gas atmospheres. The samples were fractured in vacuum and high resolution XPS spectra of the fractured surfaces were measured. The peak positions and peak shape parameters of Fe 3p for Fe2+ and Fe3+ were derived from the Fe 3p XPS spectra of the standard samples of 2FeO·SiO2 and Fe2O3, respectively. Using these parameters, the Fe 3p peaks of Fe3O4 and Fe1−yO are analysed. The results indicate that high resolution XPS techniques can be used to determine the Fe2+/Fe3+ ratios in metal oxides. The technique has the potential for application to other transition metal oxide systems.