SiO2/GaAs interfaces have been obtained by deposition of Si on oxidized GaAs(110) surfaces, with the reduction of As and Ga oxides to form Si oxide. High resolution photoemission studies have been performed to elucidate the chemical state of the atoms liberated from the oxide. The reduction of As oxides, faster than that of Ga oxides, produces an As enrichment at the interface, that creates deviations from GaAs stoichiometry. This As excess appears in the high resolution spectra as a component with an energy shift between 0.45 and 0.65 eV from the GaAs substrate signal. Subsequent reduction of Ga oxides produces the decrease of the component related to As excess. In any stage of the process no segregation of Ga appears from the spectra. Finally the As-excess component vanishes, showing that it reacts with the liberated Ga atoms to form again GaAs. A quantitative analysis of the photoemission intensities has allowed the establishment of a structural model of the atomic arrangement during the chemical changes.