Synchrotron radiation photoemission spectroscopy (PES) has been used to study thermal SiO2/Si(100) interfaces. Oxides were grown at 700, 800, 900, and 1000 degrees C and etched back to a thickness similar to 0.5-1.5 nm for PES measurements. Comparison of Si 2p core levels with in situ ultrahigh vacuum-grown SiO2 on Si(100) indicated that various Si oxidation states, Si-x+, are present at the interface. The results show that the amount of both Si2+ and Si3+ increases with increasing oxidation temperature while the amount of Si1+ remains constant. For these furnace grown oxides, structural relaxation to relieve strains rather than kinetic growth considerations governs the structure and suboxide distribution at the SiO2/Si(100) interface.