Chemical analysis of residual SiO 2 layers by X-ray photoelectron spectroscopy (XPS) was used for the characterization of thermal oxide removal from Si(1 0 0) substrates in process gas environments applicable to metal-organic vapor phase epitaxy (MOVPE). A dedicated transfer from the MOVPE reactor to UHV allowed us to obtain undistorted XP spectra without any contamination or reoxidation of the Si(1 0 0) surfaces. Small photoelectron take-off angles provided enhanced surface sensitivity. Based on the relative evaluation of the Si2p peak intensities associated with different chemical coordinations, we derived the average residual oxide thicknesses from the XPS results in order to evaluate different annealing procedures. In dependence of basic process parameters such as annealing temperature, we found a rather abrupt deoxidation behavior and defined a critical surface temperature for complete oxide removal for certain compositions of the process ambient. In purified hydrogen, a lower reactor pressure slightly enhanced oxide removal, while the addition of the precursor silane largely promoted deoxidation. Although nitrogen is a common alternative process gas, silicon nitrides formed during annealing before the protective oxide layer was completely removed. In contrast, we verified oxide removal in argon, where an increased critical surface temperature was observed.