The formation of SiO2/GaAs structures from reduction of substrate oxides has been studied by x‐ray photoemission spectroscopy. GaAs oxides reduction is induced by Si deposition and subsequent high‐temperature annealing. The deposition of Si on oxidized GaAs surfaces promotes the reduction of the GaAs oxides and the formation of SiO2, Si diffusion being the predominant process. This mechanism allows one to obtain SiO2thicknesses up to 15 Å. Over this limit Si diffusion is negligible, Si nucleates on top of the Si dioxide layer, and a substantial amount of GaAs oxides can remain unreduced between the SiO2layer and the GaAs substrate. Subsequent annealing to 870 K produces the disappearance of the GaAs oxides together with loss of Si atoms. A process based on breakage of the Ga–O bonds with Ga atoms remaining between SiO2and the substrate, and oxygen atoms diffusing through the SiO2to the outer surface, is consistent with the experimental results. The oxygen atoms react with the outer Si layer, showing an etchant behavior, and the Ga and As atoms supplied by their oxides recombine to form GaAs.