Employing Si 2p and O 1s photoemission spectroscopy using monochromated synchrotron radiation and the supersonic molecular beam technique, we have performed real time in situ observations of oxidation states on Si(001)-2 x 1 at room temperature. High-resolution Si 2p photoemission spectra, which unambiguously resolve oxide components [Si1+, si(2+), Si3+ and Si4+], were successfully measured requiring only 43 s per spectrum. We found that the Si4+ species gradually increases to reach the oxide thickness of 0.57 nm just after the saturation of Si1+, Si2+ and Si3+, species with a translational energy of 2.9 eV. (C) 2002 Elsevier Science B.V. All rights reserved.