The temporal evolution of the electrically active B fraction has been measured experimentally on B implanted Si, and calculated using atomistic simulation. An implant of 40 keV, 2×1014 cm−2 B was examined during a postimplant anneal at 800 °C. The results show a low B activation (∼25%) for short anneal times (⩽10 s) that slowly increases with time (up to 40% at 1000 s), in agreement with the model proposed by Pelaz et al. [Appl. Phys. Lett. 74, 3657 (1999)]. Based on the results, we conclude that B clustering occurs in the presence of a high interstitial concentration, in the very early stages of the anneal. For this reason, B clustering is not avoided by a short or low-temperature anneal. The total dissolution of B clusters involves thermally generated Si interstitials, and therefore, requires long- or high-temperature anneals.