A scanning tunneling microscopy and spectroscopy study of the recombinative desorption of hydrogen from the Si(100)-2×1 surface is presented. Initially at room temperature, hydrogen atoms singly occupy the Si dimer units while at higher temperatures (630 K) these atoms tend to pair up. This pairing phenomenon is due to a -bonding interaction which favors pairing individual dangling bonds on the dimer units. Following desorption from the saturated surface, pairs of dangling bonds are found loaalized on the Si dimer units. The significance of these paired-dangling-bond sites is d)scussed in ligh* of the desorption model recently proposed by Sinniah et al. © 1991 The American Physical Society.