Ta2O5-x films prepared on Si(100) by chemical vapor deposition and annealed in oxygen gas, both natural abundance and isotope labeled [O-16(2)(77%)+O-18(2)(23%)], were analyzed by in situ x-ray photoelectron spectroscopy and ex situ time-of-flight secondary ion mass spectroscopy (TOFSIMS). Before annealing at 800 degrees C, the film was oxygen deficient, i.e., Ta2O5-x, and there was a very small amount of interfacial SiOy. During annealing in O-2, additional Si was oxidized and the Ta2O5-x approached stoichiometric composition. Based on TOFSIMS depth profiles of O-18-labeled ions, Si at the interface is oxidized during O-2 annealing mainly by transfer of O from Ta to Si, not by migration of gas phase O-2 through Ta2O5-x to the unoxidized Si. The atomic oxygen from the dissociative reaction of gas phase O-2 is transported through Ta2O5-x, by a vacancy mechanism and replenishes the loss of O from Ta2O5-x to Si.