The profile of oxygen atoms diffused from SiO2 films into Si substrates as interstitials during high temperature annealing has been studied by infrared absorption spectroscopy using 18O as a tracer. The measured absorbance profile in the substrates can be explained in terms of a simple diffusion model in which the SiO2 layer acts as a reservoir of O and there is no energy barrier to diffusion across the Si/SiO2 interface. The driving force for diffusion is the temperature‐dependent solubility of oxygen interstitials in the Si. We find evidence both for the outdiffusion of oxygen from the SiO2 layer into the Si during extended annealing and retrodiffusion back into the SiO2 layer during temperature ramp down. From etchback profiling data on O interstitials we are able to revise the Arrhenius law for O diffusion in Si and obtain an activation energy of 2.57 eV and a preexponential factor of 0.22 cm2 s-1. It is further suggested that there is evidence for enhancement of the O diffusion coefficient in Si at low temperatures resulting from the presence of foreign species such as H. These observations are important in understanding the structure of a thermally grown SiO2/Si interface and annealing‐induced degradation in Si‐based devices. © 1996 American Institute of Physics.