Adsorption of B induces a (3 × 3) R30°reconstruction on Si(111). By combining scanning tunneling microscope topographs and spectra with first-principles calculations we are able to follow the different stages of B incorporation in the Si surface and the corresponding changes to the surface electronic states. We find that the thermodynamically stable configuration consists of a B substitutional atom directly below a Si adatom at a T4 site. The stability of this configuration is due to the relief of subsurface strain by the short B-Si bonds and the passivation of the surface obtained through charge transfer from the Si adatom to the substitutional B. © 1989 The American Physical Society.