A structural model for the Al/SiO2 interface system after an oxidation-reduction reaction is proposed. Thermal dependence of the initial stages of the Al/SiO2 interface formation process is investigated by in situ x-ray photoemission spectroscopy analysis and secondary ion mass spectrometry measurements of depth profiles. An abrupt interface without any reaction is confirmed when the interface is formed at room temperature. During Al deposition on SiO2 at substrate temperatures beyond 300 degrees C, oxidation-reduction reaction occurs that produces Al2O3 and reduces metallic Si at the interface. The reduced Si spreads throughout the entire Al layer uniformly and the thin Al2O3 layer acts as a diffusion barrier that limits the reaction in the very vicinity of the interface. This suggests a distinct layer ordering of the reacted system, Al(Si)/Al2O3/SiO2.