Diamond-silicon carbide composites were sintered from diamond powder and liquid silicon at high pressure-high temperature (HPHT) conditions. Experiments were conducted in the diamond-stable region and then repeated in the graphite-stable region. X-ray diffractograms of the specimens sintered for different time periods provided information on the SiC formation rate and activation energy. Only the late stage of the reaction was investigated, and in the diamond-stable region it was shown that SiC growth was controlled by the diffusion rate of silicon and carbon atoms though the existing layer of SiC. This process is characterized by an activation energy of 264 kJ/mol. At 2 GPa, where graphite is the stable form of carbon, in addition to the direct reaction, diamond may first spontaneously transform into graphite, which next reacts with silicon. A combination of these two processes results in a higher activation energy of 410 kJ/mol.