The kinetics of the nitridation reaction of silicon powder compacts in a flowing 5% hydrogen/95% nitrogen stream over a temperature range of 1448-1648 K was investigated by thermogravimetry (TG). The effects of gaseous flow rate, pellet size, grain size, nitrogen concentration and reaction temperature on the nitridation reaction were extensively studied. The reaction order of the nitridation is found to be 0.71+/-0.03, and the activation energy of the reaction is 292+/-5 kJ mol(-1). A reaction mechanism, which includes the role of hydrogen in the nitridation system, is proposed to explain the experimental observations. The nitridation of a silicon pellet is mainly controlled by the diffusion of gaseous reactants through the product layer. A kinetic model of the nitridation process is developed which agrees well with the experimental results.