HfO2/SiO2/Si(001) structures were annealed in dry oxygen, and compositional depth profiles were measured by high-resolution Rutherford backscattering spectroscopy (HRBS). Growth of the interfacial SiO2 layer and simultaneous surface accumulation of Si were observed. The observed result indicates that silicon species are emitted from the SiO2/Si interface to release the stress induced by oxidation as was predicted by theoretical studies. The behavior of oxygen during the annealing was also observed using 18O as a tracer. The observed 18O profile suggests that oxygen molecules are decomposed into atomic oxygen in HfO2 and diffuse through the oxide layer via exchange mechanism. This is also supported by the observed activation energy of ∼ 0.6 eV for the growth of the interfacial SiO2 layer.