The physical nature of the electrically inactive phosphorus in silicon was investigated by annealing experiments performed on laser annealed specimens doped by ion implantation up to 5×1021 at/cm3. The hypothesis of point defects, which compensate or make the excess dopant electrically inactive, is contradicted by the experimental results. It was verified that phosphorus solubility corresponds to the electrically active concentration in equilibrium with the inactive dopant, and that the latter is precipitated phase. This was confirmed by transmission electron microscopy (TEM) examinations with the weak beam technique, which detected a high density of very small coherent precipitates. This method allowed us to observe particles of the same kind even on specimens thermally predeposited in conditions typical of device technology. In both cases the amount of precipitates was consistent with the inactive dopant concentration. In addition these experiments show that precipitation is associated with a high enhancement of the diffusivity of the dopant. This phenomenon can account for the plateau region of the carrier profiles after thermal predeposition.