Kinetic Monte Carlo simulations have been used to investigate mechanisms for boron clustering in crystalline and preamorphized Si. We have extended previous boron-interstitial cluster models to include larger and more stable complexes in order to reproduce boron cluster evolution at very high boron concentrations. We have investigated the stoichiometry of boron-interstitial clusters resulting from low temperature recrystallization of preamorphized layers. We have performed a dedicated experiment based on boron implanted into preamorphized Si with end-of-range defects placed far enough from the boron profile to avoid the interaction between end-of-range defects and resulting boron-interstitial clusters after recrystallization. Hall measurements on active B dose combined with a systematic analysis performed by Kinetic Monte Carlo simulations indicate that initial boron-interstitial clusters after recrystallization …