Bulk crystals of Si0.4Ge0.6, grown using the vertical Bridgman method, were oxidized at 800 °C for 1 h using oxygen in dry oxidation and a stream of de-ionized water in wet oxidation. Single-like and polycrystalline Si0.4Ge0.6samples were used. The oxidized samples were analyzed using x-ray photoelectron spectroscopy to determine the chemical states of the Si and Ge in the oxide. In the sample oxidized using a dry ambient, the oxide layer consisted mainly of SiO2, and GeO2 formed the subsurface area only. This can be accounted for by the differences in the formation of the free energies of the SiO2 and GeO2. In wet oxidation, the oxide layer had a mixed structure consisting of SiO2 and GeO2. To understand the oxide formation mechanisms in the wet oxidation process, an introduction of a chemical bond manipulation explained the formation of the mixed structure.