We investigated Si surfaces after annealing in a H-2 ambient using attenuated total reflection in the infrared region, reflective high energy electron diffraction, Auger electron spectroscopy, and atomic force microscopy. We found that at all H-2 pressures the surface dangling bonds formed dimers that. were related to two-domain (2x1) or c(4x2) reconstructed surfaces. H-2 was adsorbed on the reconstructed surface and terminated a pair of dangling bonds that did not form dimers. H-2 adsorption was limited by the reaction between H-2 and the dangling bonds on the surface. The activation energy of H-2 adsorption was 0.4-0.6 eV higher than that of H-2 desorption. The surface on which H atoms were adsorbed and dimers were formed was inert, which kept the surface clean. We also found that the H-2 annealed surfaces were influenced by surface roughness and contaminants including oxygen and carbon.