Variations of the XPS spectra of Fe1−xO crystal and iron metal oxidized in air were studied when the samples were heated in a vacuum. The structural variations observed by XPS can be explained by the phase diagram of the iron‐oxygen system. The intensity ratio of O 1sto Fe 2p3/2for Fe1−xO at 800 °C in thermal equilibrium was smaller than that of the sample quenched from 800 °C to room temperature. This indicates that quenching the sample changes the surface structure. FWHM (full width at half‐maximum) of 2.2 eV of O 1sfor Fe1−xO was larger than that of 1.7 eV for &agr;‐Fe2O3. The broader FWHM is due to the large value ofxand clustering of the cation vacancies in Fe1−xO. The binding energies and spectral profiles of Fe 3sand Fe 2p1/2,3/2for Fe1−xO coincided with the spectra for Fe2SiO4or FeTiO3having only divalent iron ions.