The chemical composition of ultrathin oxide-nitride-oxide multilayer films grown onto p-type silicon substrates and subjected to different annealing processes and to various oxidation times of the nitride layer has been studied by means of x-ray photoelectron spectroscopy and time-of-flight-secondary-ion-mass spectroscopy. Our results show that the annealing process strongly influences the bottom SiO2/Si interface allowing the saturation of the dangling bonds present at this interface and decreasing the concentration of free hydrogen. By increasing the oxidation time, a better silicon dioxide layer is formed in the topmost layer of this structure.