The purpose of this letter is to observe voids at the SiC–Si interface beneath the SiC layers grown by chemical vapor deposition at high temperature. It is shown in this letter that the volume of the voids per unit area is proportional to the oxygen concentration in the Si substrate over seven orders of magnitude.In situdynamical reflectivity measurements show that the voids are formed during the carbonization step and especially when the carbon, which has diffused deeply into the Si substrate, diffuses back toward the SiC layer just completed at the substrate surface. This back diffusion is due to the inversion of the carbon concentration gradient sign at that moment. It is accompanied by the formation of CO, resulting either from the reduction of SiO orSiO2dissolved in the Si substrate. Diffusion of carbon in silicon might improve the methods of purification for the removal of oxygen which remains the main impurity of the purest silicon material. ©1997 American Institute of Physics.