The possibility of altering an oxygen-adsorbed Si surface to SiO 2 using a synchrotron radiation (SR) excited reaction is evidenced for the first time. Oxygen gas is adsorbed on a clean Si surface, and soft x-ray is irradiated on it by SR. As a result, H partly terminated on the oxygen-adsorbed Si surface is eliminated and the surface becomes more SiO 2-like. This is proved by x-ray photoelectron spectroscopy (XPS) analysis. Photostimulated desorption (PSD) of H+ ions, which are emitted from the surface, is also detected during SR irradiation. The Si - O bond formation model followed by H+ PSD explains this oxidation.