We perform first-principles total-energy calculations to identify the stable and metastable configurations of interstitial B in Si. We calculate formation energies and ionization levels for several equilibrium ionic configurations in different possible charge states. In all charge states the ground state consists of a B atom close to a substitutional site and a Si self-interstitial nearby. The binding energy of the self-interstitial to the substitutional B is, however, rather weak, of the order of 0.2–0.3 eV. The ground state has negative-U properties in accordance with experiments. We find several charge-state-dependent metastable configurations of interstitial B energetically close to the ground state. We discuss on the basis of formation energies the role of excess Si interstitials in the activation of B diffusion and the charge-assisted transport mechanism in the activation of B diffusion.