Ta2O5 films, deposited on bare and nitrogen passivated Si(100) surfaces, were annealed in the presence of three different oxidizing agents, NO, N2O, and O-2, and then examined using X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOFSIMS). These films were thin enough to allow spectroscopic examination of the interface between the silicon and the oxide. During deposition of Ta2O5 on bare Si(100), the substrate is oxidized and the amount of oxide increases during subsequent annealing, No nitrogen accumulates when annealing in NO or N2O. Angle-resolved XPS and TOFSIMS depth profiles reveal, after annealing, oxidized silicon at the vacuum-solid interface and throughout the oxidized tantalum layer. For Ta2O5 on nitrided Si(100), annealing at 820 degrees C depletes nitrogen in the silicon nitride and increases silicon oxide, regardless of the oxidant used. Temperature programmed reaction spectra of Ta2O5 deposited on previously oxidized Si(100) show SiO desorption above similar to 950 degrees C. The annealing results are accounted for in terms of chemical reactions at the buried interface that form SiO and, for the nitrided samples, NO species that migrate toward the vacuum interface, Oxidizing species from the gas phase also migrate to the buried interface to produce additional silicon oxide.