The adsorption and dissociation ofH2Son GaAs(100) has been studied using high-resolution electron energy loss spectroscopy, thermal desorption spectroscopy, and isotope exchange techniques. The dissociative adsorption ofH2Sat 100 K produces only H–As species with a vibrational frequency of2072 cm−1.Upon warming to 200 K, the vibration of H–As clearly shifts to2105 cm−1,corresponding to a free H–As species. In addition, the formation of H–Ga(1887 cm−1) is also observed upon thermal annealing. In coadsorption studies ofH2Sand D atoms, three main desorption features ofH2Swere observed at 135, 200, and 375 K, respectively. The peaks at 135 and 375 K can be attributed to the desorption of molecularly adsorbedH2Sand the recombinative desorption of adsorbed H and HS, respectively. In addition to the 375 K peak, the desorption feature at 200 K also undergoes isotope exchange between coadsorbed D atoms andH2S.These observations strongly suggest that the dissociative adsorption ofH2Son GaAs(100) involves an intermediate of Ga–HS—H–As. ©1998 American Institute of Physics.