The depth profiling of O 1s energy loss in silicon oxide near the SiO2/Si interface was performed using extremely small probing depth. As a result, the energy loss of O 1s photoelectrons with threshold energy of 3.5 eV was found. This value of 3.5 eV is much smaller than the SiO2 bandgap of 9.0 eV, but quite close to direct interband transition at Gamma point in energy band structure of silicon. This can be explained by considering the penetration of electronic states from silicon substrate into silicon oxide up to 0.6 nm from the interface. In addition, the penetrating depth is larger than the thickness of the compositional transition layer. (C) 2002 Elsevier Science B.V. All rights reserved.