Isotopic tracing was used to determine the distribution of O and N after the final fabrication step of ultrathin silicon oxide/nitride/ oxide (ONO) films. revealing how the processing parameters affect the characteristics of the resulting structure. ONO films were prepared on precleaned Si wafers in three steps: (i) thermal oxidation in dry O-16(2), at 1000 degrees C in a rapid thermal processing furnace; (ii) remote plasma enhanced chemical vapor deposition of a nitride layer in a mixture of N-15(2) and silane, and (iii) a second thermal oxidation step in a Joule-effect heated furnace, in dry O-18(2), at three different temperatures and for three different times. Nuclear reaction analysis and narrow nuclear resonance depth profiling with nanometric resolution were used to determine the distribution of O-16, O-18, and N-15 in the samples, before and after the reoxidation step. It is shown that the final structure is not a stacked one, but rather a silicon oxynitride ultrathin film with variable composition, presenting moderate concentrations of N in the near-surface and near-interface regions, and a higher N concentration in the bulk. Long treatments at high temperatures produce a significant loss and redistribution of N.