Nitrogen was deposited on the surface of Si(100) wafers by ion implantation at a very low energy (approximately 20 eV), at fluences between 1 and 10×1014 cm−2. The samples were thermally oxidized in dry O2 at temperatures between 800 and 1050 °C. Atomic transport of the chemical species involved in the process was investigated by isotopic tracing of N, O, and Si, using depth profiling with nanometric resolution. The obtained results indicate that: (i) the nitrogen atoms deposited on the Si surface are redistributed during thermal oxidation in O2 within the silicon oxide (oxynitride) film, with maxima at the near-surface and near-interface regions; (ii) during growth, O is fixed not only in the near-interface and near-surface regions like in the thermal growth of SiO2 films on Si, but also in the bulk of the growing oxide (oxynitride) film; and (iii) Si is immobile during the thermal oxidation process. The observed modifications in the mechanisms of thermal growth of SiO2 (SiOxNy) films on Si due to the presence of N are discussed. © 1999 American Institute of Physics.