Precipitation of boron in heavily doped silicon has been investigated using transmission electron microscopy (TEM) and atom-probe tomography (APT). Si wafers were implanted with a very high boron dose (1 × 1017 at. cm−2) at 27 keV and further annealed at 500, 750 and 1000 °C for 1 h. Results show that precipitates nucleate during implantation before any annealing has been made. They were found to have a smaller boron content than that of the expected phase SiB3. With increasing thermal budget, the concentration of boron approaches the equilibrium composition 75 at.%. It is thought that small boron-enriched precipitated clusters that form in supersaturated silicon should be distinguished from the boron interstitial clusters (BICs).