The diffusion of boron in silicon with and without a 20-nm thick epitaxial CoSi2 layer was investigated by secondary ion mass spectrometry (SIMS) using doping superlattices (DSLs). For specimens without CoSi2 layer, we observed oxidation enhanced diffusion (OED) of B in Si in accordance with the literature. However, B diffusion in silicide capped specimens was found to be retarded by about a factor of 20 as compared to specimens without a silicide layer. The measured diffusivity was even less than the intrinsic thermal diffusivity of B in Si.