We investigated the transport of oxygenic and nitrogenic species during the initial stages of thermal nitridation of silicon dioxide films in ammonia by means of isotopic tracing of oxygen and nitrogen. The total amounts of the different isotopes incorporated during oxidations and nitridations under O-18(2), O-16(2), (NH3)-N-15, or (NH3)-N-14 were determined by nuclear reaction analyses, which are highly sensitive and selective. The depth profiles of the different isotopes were determined using methods based on very narrow resonances existing in the nuclear reaction cross sections at low energies. These methods lead to high depth resolution, especially near the surface, where most of the other available methods fail in providing acceptable depth resolutions. The results here reported show that during the initial stages of thermal nitridation, both oxygenic and nitrogenic species are transported toward the oxynitride/silicon interface as well as toward the surface. Oxygen atoms are lost from the oxynitride film surface, exchanged for the incorporated nitrogen atoms. A strong exchange between the nitrogen atoms incorporated into the oxynitride film and those from the ammonia gas is also observed. The dependence of the nitrogen profiles on the ammonia pressure and on the nitridation time in the initial stages indicates that the overall nitridation process is governed by diffusion mechanisms.