The redistribution of impurities during thermal oxidation of silicon was studied both theoretically and experimentally. Experiments with specific impurities indicate that gallium, boron, and indium deplete from silicon, while phosphorus, antimony, and arsenic pile up during thermal oxidation. It is shown that the redistribution process can be significantly influenced by the escape of impurities through the oxide layer as well as by the segregation of the impurity at the oxide‐silicon interface.