Using an ab initio method we analyze the mechanisms of the boron diffusion with emphasis on the role of the intrinsic interstitials. It is shown that the boron diffusion is dominated by a kick-out mechanism. The different effect of silicon and carbon interstitials gives rise to kinetic effects. A preference for a kick-in of the boron interstitial into the carbon lattice sites is found. Kinetic effects reported in co-implantation experiments and in-diffusion experiments are explained by our findings.