The mean escape depths of Si 1s, Si 2p, and O 1s photoelectrons from silicon−silicon dioxide structures were determined over a wide kinetic−energy range with magnesium, aluminum, and chromium x−ray sources. Variations in photoelectron line intensity were measured as a function of oxide film thickness. Film thicknesses were determined from ellipsometric data assuming an index of refraction of 1.47 for the oxide. Discrepancies in fitting intensity data to an escape−depth curve for films <20−Å thick are associated with nonstoichiometry of the oxide film near the interface. The incompletely oxidized interfacial region is identified by variations in linewidth and in binding energies of the spectral lines as well as by the deviations from predicted intensity data.