We present the results of an experimental investigation of initial thermal oxidation of the Si(001) surface at the surface temperature of 860 and 895 K in the O-2 pressure of 1 x 10(-4) Pa. The time evolution of Si oxidation states was measured by real time in situ Si-2p photoemission spectroscopy with high energy-resolution synchrotron radiation. The changes of each oxidation state (Si1+, Si2+, Si3+ and Si4+) were clearly monitored. The Si4+ oxidation state was observed even in the early stage of oxidation after the appearance of Si3+ Species. These oxidation states have appeared in order of the low oxidation number. The onset of Si4+ species at 895 K appeared quickly comparing with that at 860 K, indicating the fast diffusion process of adsorbed oxygen atoms. (C) 2003 Elsevier Science B.V. All rights reserved.