The valence-band and O 2s core-level spectra of ultrathin (about 1 nm) SiO2 layers formed in the initial stages of the oxidation of Si(100) substrates were measured by high-resolution X-ray photoelectron spectroscopy (XPS), and the energy difference between the bonding states in the valence-band and the core-level was found to be larger than the corresponding difference for the bulk SiO2. The energy difference between the top of the valence-band and the core-level was also found to be larger than that for the bulk SiO2. From the first-principle molecular orbital (MO) calculations for SiO2 model clusters, (Si5O16H12), it was concluded that the atomic structure of SiO2 at the SiO2/Si interfaces is characterized by a narrow intertetrahedral bond angle, about 135 degrees.