The photochemistry of Si2H6 adsorbed on a hydrogen terminated silicon surface and the subsequent reactions of the photolysis products were investigated using high resoln. electron energy loss spectroscopy and by measuring time-of-flight distributions with a mass spectrometer. The cracking pattern of the products ejected directly into the gas phase without colliding with either the surface or other mols. indicates that the primary photolysis channels yield mostly fragments that contain one silicon atom. It is likely that silicon is added to the surface by insertion of SiH2 radicals into Si-H bonds at the surface but there is little evidence for reactions that remove excess hydrogen from the surface at 110 K.