A scanning-tunneling-microscopy study of the structures produced by the adsorption of atomic H on the Si(100) surface is presented. Moderate exposures at room temperature (RT) produced the 2×1 monohydride phase. Saturation exposures at 370 K produced the mixed 3×1 phase which, following an additional RT exposure, slowly reverted to the bulklike 1×1 dihydride phase. Saturation exposures at RT produced the 1×1 dihydride phase indicating that the 3×1 phase is a H-induced reconstruction which exists only under special conditions. The etching of the Si(100) surface by H atoms was also observed and its implication for saturation coverage and the species present on the surface are discussed. © 1990 The American Physical Society.