In a series of experiments, the influence of phosphorus diffusion at high concentrations on the diffusion of an antimony marker layer was investigated. The marker layer was separated from the surface by a 4 &mgr;m layer of epitaxially grown silicon. To reduce the effects of implantation enhancement and phosphorus precipitation on the diffusion of the antimony marker layer, the phosphorus was implanted into a polysilicon layer deposited on top of the single‐crystalline substrate. It was found that the diffusion of the antimony marker layer is already reduced by the epilayer. From these diffusion coefficients, upper limits for the fractional diffusivity of antimony were derived. In contrast to previous investigations, the diffusion of the antimony marker layer was found to be enhanced below regions where phosphorus was implanted. A comparison of diffusion in FZ and CZ samples shows that this enhanced diffusion of antimony can be explained only by an injection of self‐interstitials from the phosphorus‐doped region. Since the polysilicon layer was found to recrystallize, this self‐interstitial injection can be the result of phosphorus diffusing mainly via self‐interstitials, phosphorus precipitation, or both. Using Boltzmann–Matano analysis and antimony diffusivity data, the fractional diffusivity of phosphorus via self‐interstitials was estimated to be lower than 0.71 at 950 °C. Possible sources of error in this estimation are discussed. ©1995 American Institute of Physics.