Buried oxides in silicon‐on‐insulator (SOI) structures have been reported to grow in thickness by thermally oxidizing the superficial silicon layer. A thermodynamic model is presented which is based on the experimental observation that thermal oxidation leads to an increase of the oxygen interstitial concentration in silicon beyond its normal solubility. This increase is assumed to be proportional to the growth rate of the external thermal oxide. Discrepancies between the only two available sets of data are discussed in terms of different levels of oxidation‐induced supersaturations of silicon self‐interstitials. ©1995 American Institute of Physics.