In this work we present a scanning tunneling microscopy and spectroscopy study of the chemistry of H on the Si(111)-7 × 7 surface. We show that the structure and nature of the bonding at the surface controls the reactivity and determines the types of hydride species present on the surface. The reactivity is dominated by the relaxation of the strained bonds formed by the reconstruction. The limited number of such bonds accounts for the apparent saturation of H on this surface. Both the kinds of hydrides present on the surface and their relative populations are determined by the manner in which this relaxation occurs. At low temperatures, the strain in the adatom backbonds is relaxed by the formation of adatom trihydride species while at higher temperatures relaxation occurs via the formation of bulk-like adatom islands. The strained restlayer can only be effectively relaxed at higher temperatures and results in the formation of a 1×1 structure. In all, the surface structure attempts to approximate that of the unstrained bulk. Desorption of H2 from this surface results in the rearrangement and the ultimate breakup of the adatom islands, the adatoms they contain are redistributed over the surface, eliminate H from the restlayer, and result in the re-establishment of a 7×7 adatom structure. © 1991.