The intrinsic nitridation kinetics of high-purity silicon solid to form silicon nitride were determined by thermogravimetric analysis technique. The initial rate method was used to avoid kinetics falsification due to intraparticle diffusion limitation caused by the formation of silicon nitride layer around the silicon particles. Relatively small sample sizes were used to eliminate the interparticle diffusion limitation (compact effect) that becomes important as the reaction proceeds, owing to the expansion of the individual particles and filling of the initial pores within the compact. The effect of hydrogen addition to the nitriding gas, as well as the influence of the flow rate, on the reaction progress was also investigated.