The thermal desorption of thin oxide films on n- and p-type InP has been studied by using x-ray photoelectron spectroscopy. It was found that oxides formed on InP could be completely desorbed at 458 °C without causing surface decomposition. The phosphorus and indium oxides desorbed together within 5°. However, heating the oxidized surface to about 453 °C, while not changing the chemical structure of the phosphorus oxides, did remove a significant amount of indium oxides. Heating at this lower temperature also induced changes of the position of the surface Fermi level. On n-InP, it changes from 0.1 eV below the conduction-band minimum to 0.3 eV, and on p-InP, from 0.6 to 0.4 eV. Upon complete desorption of all surface oxides at 458 °C, the Fermi level on n-InP was then pinned at 0.3 eV, whereas that of p-InP moved down to 0.9 eV. This suggests a reduction of donor defects. Heating the surface of 458 °C for more than a few minutes or heating at a slightly higher temperature caused a loss of phosphorus. On such a decomposed surface, the Fermi level of n-InP was pinned at 0.5 eV and p-InP at 0.7 eV, which indicates the formation of more donor and acceptor defects.