<p>The influence of the degree of preliminary graphitization of synthetic diamond powder on the efficiency of silicon carbide (SiC) formation on its surface during high-temperature silicidation by SiO gas was investigated. It is shown that the original and weakly graphitized diamond powders practically do not react with SiO at 1400 °C due to their high chemical inertness caused by sp³ hybridization of carbon. In the case of highly graphitized samples, the formation of nanocrystalline SiC was observed predominantly in areas where graphite layers formed on the surface of diamond particles. The average size of SiC crystallites was 15–20 nm. The obtained results demonstrate that preliminary graphitization is a necessary step for efficient silicidation of diamond powders with gaseous SiO.</p>