Thermogravimetry, differential thermal analysis, mass spectrometry, and X-ray diffractometry were used to study the reaction process of the in situ reaction between Si3N4, B4C, and carbon for the synthesis of silicon carbide-boron nitride composites. Atmospheres with a low partial pressure of nitrogen (for example argon + 5%-10% nitrogen) seemed to inhibit denitrification and also maintain a high reaction rate. However, the reaction rate decreased significantly in a pure nitrogen atmosphere. The experimental mass spectrometry results also revealed that B4C in the Si3N4-B4C-C system inhibited the reaction between Si3N4 and carbon and, even, the decomposition of Si3N4. The present results indicate that boron could be a composition stabilizer for ceramic materials in the Si-N-C system used at high temperature.