The angle-resolved Si 2p photoelectron spectra arising from the transition layers formed on both sides of the SiO2/Si(100) interface covered with more than one SiO2 monolayer, which is device grade, were measured with the probing depth of nearly 2 nm. The novel analytical procedure of these spectra is developed by considering that one SiO2 monolayer, two compositional transition layers (CTLs), and one Si monolayer on Si substrate surface are continuously connected with each other. It was found for the interface formed in dry O2 at 900 °C that two CTLs constitute the first and second CTLs. The first CTL formed at the interface consists mainly of Si1+, Si2+, and Si0 and the second CTL formed on the first CTL consists mainly of Si3+ and Si4+. Influences of oxidation temperature, annealing in forming gas on the chemical structures of transition layers formed on both sides of the interface are also discussed.