We investigated the transport of oxygenic and nitrogenic species during thermal nitridation of silicon dioxide films in ammonia in a resistance heated furnace, as well as during subsequent reoxidation of the resulting oxynitride films. Isotopic tracing of oxygen and nitrogen was used, showing that the thermal nitridation of SiO2 films during long time intervals (around 1 h) is driven by essentially the same mechanisms as in the initial stages : the transport of NH3 by interstitial diffusion toward the oxynitride silicon interface reacting with the silicon network as well as toward the surface where it desorbs. There is a general enhancement of the effects previously observed in the initial stages of nitridation, namely, the amount of nitrogen accumulated in the SiO2/Si interface is greatly enhanced. Thermal reoxidation in dry O-2 of the oxynitride films implies en increase of the film thickness. The isotopic tracing of oxygen and nitrogen during thermal reoxidation shows the incorporation of freshly arriving oxygen mainly in the surface and in the oxynitride/silicon interface regions. Nitrogen atoms near the surface and near the interface are exchanged for oxygen atoms introduced during the reoxidation. A general discussion of the mechanisms of thermal nitridation of SiO2 films in NH3 and the symmetry between the mechanisms of thermal nitridation and of thermal reoxidation in dry O-2 of the resulting oxynitride films is presented.