The effect of additives is investigated for the carbothermal reduction synthesis of nanophase silicon carbide/silicon nitride composite powders. Mixtures of silica, carbon, seed silicon nitride, and additive are reacted in a thermogravimetric analyzer. The mass loss information combined with compositional and spectroscopic analysis allows product quality (morphology, surface area, alpha -Si3N4 and beta -SiC contents, oxygen content, etc.) information to be obtained. It was observed that all of the additives used in this study increased the reaction rate. Lithium carbonate produced a silicon nitride/silicon carbide composite that was not significantly different from experiments without promoter. However, the product quality was severely affected in other instances.