Here we present some results of transmission electron microscopy and secondary ion mass spectroscopy of thermally oxidized gallium arsenide with different types of dopants. At temperatures below 400 °C an amorphous oxide is formed. Oxidation at temperatures between 500 and 600 °C initially produces an epitaxial film of γ‐Ga2O3. As the reaction proceeds, this film becomes polycrystalline and then transforms to β‐Ga2O3. This film contains small crystallites of As2O5and As2O3in the case of the chromium doped samples, whereas only the former was detected in the case of silicon and tellurium doped samples. Elemental arsenic was always found at the interface between the oxide and GaAs. Chromium doped gallium also exhibited a slower oxidation kinetics than the other materials.