Using the atomic layer deposition technique, 1.2nmAl2O3 films were deposited as high-k gate dielectric layer on hydrogen-terminated silicon and annealed in vacuum and pure hydrogen in order to elucidate the effects of growth and annealing on the structure of film, interface, and surface. As analytical tools, high resolution core level spectroscopy using synchrotron radiation as variable photon source and Fourier Transform Infrared absorption spectroscopy in the attenuated total refraction mode were employed. For Al2O3 on H-terminated Si(111) and (100) surfaces the Si–H bonds are preserved at the interface, while Si–O–Al bonds provide the atomically abrupt interface between Al2O3 and Si. The chemical and structural integrity of the interface is maintained upon annealing except for a gradual loss of Si–H bonds. Growth of a SiO2 layer is observed after annealing, that is unambiguously located at the Al2O3 surface and not at the interface. Stress-induced emission of Si atoms from the interface is identified as the source of SiO2 based on a substantial broadening of the Si 2p core lines. A thermally induced reaction between Si and Al2O3 to form volatile SiO and Al2O is suggested to be responsible for the significant thickness reduction of Al2O3 that accompanies ann- ealing at temperatures of 750°C. Conclusions for the likely effects of forming gas anneals on Al2O3/Si are drawn from this work.