Carbon diffusion into silicon is well behaved and does not generate any nonequilibrium point defects. We show that, in contrast, the diffusion of carbon incorporated in silicon well above its solid solubility will cause an undersaturation of silicon self-interstitials, which in turn may cause retarded diffusion of boron. In addition, we predict that due to this undersaturation, the diffusion of built-in carbon spikes will lead to strongly non-Gaussian concentration profiles. ©1998 American Institute of Physics.