X-ray photoelectron spectroscopy (XPS) has been used to investigate the Si(100) surfaces exposed to 02 at room temperature and subsequently heated to various temperatures ranging from 200°C to 800°C. Each 01s spectrum observed was deconvoluted into two Gaussian curves with peaks at (531.8 ± 0.3) eV and (533.6 ± 0.3) eV. The surface density of high binding energy component (533.6 ± 0.3) eV decreased with increasing temperature up to 400°C. On the other hand, the surface density of low binding energy component (531.8 ± 0.3) eV increased with increasing temperature up to 400°C. Furthermore high binding energy component increased as a result of heating at a high temperature above 400°C, and all of the oxygen adsorbed were removed from the surface by heating at temperatures above 750°C. The change of the surface density of the two components with temperature was discussed in relation to the bonding states of oxygen to silicon surfaces.