A thermodynamic model is derived which describes the growth kinetics of the Buried Oxide (BOX) layer in Silicon On Insulator (SOI) structures due to the oxidation of the superficial silicon layer. The model is based on the assumptions that this oxidation induces a supersaturation of interstitial oxygen in the superficial silicon layer and that this supersaturation is proportional to the growth rate of the external thermal oxide. We compare the model with the two data sets available and discuss the discrepancies in terms of different supersaturations in the superficial silicon layer induced by thermal oxidation.