We studied the mechanisms of thermal growth of silicon nitride films in ammonia using a resistance heated conventional furnace and a halogen lamp heated rapid thermal furnace, The methods are based on isotopic tracing of nitrogen and hydrogen using different kinds of isotopically enriched ammonia for the thermal growth, (NH3)-N-14, (NH3)-N-15, and (ND3)-N-14. The total amounts of the different isotopes were measured by nuclear reaction analyses. The nitrogen and hydrogen profiles were measured by either nuclear resonance depth profiling or nuclear reaction analyses associated with step-by-step chemical etching. A pronounced exchange between the hydrogen atoms on the silicon nitride films and those in the nitriding gas was observed, as well as an exchange of nitrogen atoms on a smaller scale. The results of the present investigation indicate that the atomic transport through the growing nitride film occurs via nitrogenous species that include hydrogen. The transport of silicon atoms or ions can also participate in the growth process, but they cannot be the only mobile species.