The oxygen exchange reactions can give valuable information concerning the oxidation mechanisms since they can be intermediate reactions. During dry thermal oxidation of silicon there are at least two different oxygen exchange reactions: oxygen exchange between oxygen molecules (O-2 <----> O-2, catalyzed by the SiO2) and oxygen exchange between oxygen from the gas phase and the oxygen in SiO2 (O-2 <----> SiO2). The O-2 <----> SiO2 exchange takes place at the surface and more surprisingly also at the SiO2/Si interface. It has been shown that the oxygen exchange rate at me Si/SiO2 interface is at least 25% of the oxygen uptake rate, requiring a movement of oxygen both from the surface to the interface and from the interface to the surface. In this study, we evaluate the oxygen exchange for different pressures and temperatures. The amount exchanged depends marginally on these parameters and can probably be taken as a measure of the thickness of the reacting layer. The chemistry of this reacting layer plays a pronounced role in the oxidation of silicon, since it sets one of the two boundary conditions for diffusion.