The rates of oxidation of SiGe and of Si covered with a thin ‘‘marker’’ of Ge have been measured, and compared with rates of oxidation for pure Si, both for wet and dry ambient. It is shown that the presence of Ge at the SiO2/Si interface increases the rate of wet oxidation by a factor of about 2.5, while it does not affect the rate of dry oxidation. By decreasing the partial pressure of H2O sufficiently, the rate of wet oxidation can be decreased to match that of dry oxidation. In this case again, Ge has no effect on the rate. Contrary to what has been proposed before, Ge is being piled up at the interface both for fast and slow oxidation. We demonstrate that the role of Ge is to suppress the formation of Si interstitials and that this is the rate limiting step in cases of rapid oxidation. For slower oxidation, interstitials have considerably more time to diffuse away and thus their formation and/or diffusion is not rate limiting.