We investigated the transport of hydrogenous species during thermal nitridation of silicon dioxide films in ammonia by means of isotopic tracing of hydrogen. The dependence of the amount of hydrogen incorporated in the oxynitride films on the nitriding temperature and time, ammonia pressure, and on the initial oxide thickness was determined using methods that allow discrimination between the incorporation of hydrogen in the oxynitride films during nitridation and the effect of hydrogen adsorption during exposure of the oxynitride films to air. The depth profiles of hydrogen and the number of hydrogen atoms that are exchanged between the oxynitride films and the ammonia gas are also established The results indicate that the nitridation is driven by a mechanism whereby ammonia diffuses toward the oxide/Si interface reacting with the silica network. The nitridation of the surface and of the interface creates diffusion barriers whose effect on the nitridation process, as well as on the incorporation of hydrogen in the oxynitride films, is discussed.