We investigate the atomic structure of B-P pairs in bulk Si through first-principles pseudopotential calculations, and find that a B-P pair is energetically most stable when P is positioned at a substitutional site in the second nearest neighborhood of B. We also examine the energetics of various I-s-B-P complexes that consist of B. P, and Si self-interstitial (I-s). In this complex, P also prefers a substitutional site in the second nearest neighborhood of B: however. its actual position depends oil the charge state of the complex. We find that the formation of the I-s-B-P complex is energetically more favorable than for the I-s-B complex, which is a major diffusing species in B-doped material.