Using a variable-temperature, ultrahigh vacuum scanning tunneling microscope (STM), we have induced and imaged the dissociation of H2S and D2S on Si(111)-7x7. H2S and D2S adsorb dissociatively at low coverage, from 50 to 300 K. Individual HS (or Ds) fragments can be further dissociated with the STM at low temperatures without affecting neighboring adsorbates. The hydrogen (deuterium) atom either desorbs or re-attaches to a nearby silicon atom. Near room temperature (297 K) and above, DS dissociates thermally, with an activation barrier of 0.73+/-0.15 eV. The activation barrier was calculated from atomistic studies of the dissociation rates at temperatures between 297 and 312 K.