We present the first microscopic calculations of the energetics of impurity-defect reactions and provide a detailed picture of the diffusion mechanisms of dopant impurities in Si. We find that vacancies mediate impurity diffusion via impurity-vacancy pairs. Self-interstitials mediate diffusion by ejecting substitutional impurities into interstitial channels and/or via impurity-self-interstitial pairs. The predicted activation energies for P and Al agree well with measured values in both intrinsic and extrinsic Si. © 1985 The American Physical Society.