A mathematical model to describe the reaction-bonded synthesis of silicon nitride is described. The process involves formation of a network of whiskers and flakes that progressively detach from the reacting solid silicon particles and cause product densification and macropore filling. In turn, this hinders nitrogen diffusion into the bulk of the compact and slows the nitridation rate at higher conversion levels. Based on the particle-pellet concept for noncatalytic gas-solid reactions, the model accounts for diffusion in both the individual particles and the macropores, as well as fracture of product layer from volume expansion. A comparison between model predictions and experimental data for different reaction conditions is made. This comparison leads to a better understanding of the structural changes (porosity and pore size) that occur during the silicon nitridation process.