We investigated the thermal decomposition of native oxide on Si(100) under ultrahigh vacuum using high‐resolution x‐ray photoelectron spectroscopy (XPS). The native oxide was formed by wet chemical treatment (HCl/H2O2/H2O), a widely employed procedure for preparing atomically clean surfaces. XPS measurements revealed that high temperature heating (≳700 °C) leads to a remarkable alteration in Si 2pand O 1sspectra. After heating to 700 °C, the Si3+structure increases and the O 1sfull‐width‐at‐half‐maximum decreases. After heating to 800 °C, the Si4+and O 1sintensity decreases but the Si2+intensity remains almost unchanged. We suggest that the formation of volatile SiO is related to the Si3+structure produced by high temperature annealing.