ABSTRACTUltra-thin SiO2/Si(111) interfaces have been studied by high resolution x-ray photoelectron spectroscopy. The deconvolution of the Si 2p core-level peak reveals the presence of the suboxide states Si3+ and Si1+ and the nearly complete absence of Si2+. The energy shifts found in the Si 2p and O is core-level peaks arising from charging effects arc carefully corrected. The valence band density of states for ultra-thin (1.8 - 3.7 nm thick) SiO2 is obtained by subtracting the bulk Si contribution from the measured spcctrum and by taking into account the charging effect of SiO2 and bulk Si. Thus obtained valence band alignment of ultra-thin SiO2/Si(111) interfaces is found to be 4.36 ± 0.10 eV regardless of oxide thickness.