The electronic structure of thermally oxidized tungsten used as an emitter in thermal ionization of organic molecules is studied. Tungsten foil was thermally oxidized at oxygen pressure 1 Torr and temperature 950 K. The photoemission spectra from the valence band and O 2s and W 4f core levels are studied under synchrotron excitation with the photon energies 100 ÷ 600 eV. It is shown that thermal oxidation of tungsten leads to the formation in the W near-surface region various tungsten oxides with an oxidation state from 6+ to 4+. In this case, mainly tungsten oxides with an oxidation state of 6+ are formed on the surface, the proportion of which gradually decreases with distance from the surface with an increase in tungsten oxides with an oxidation state of 4+.