The structure and properties of adsorbates on GaAs surfaces are investigated. The vibrational states of adsorbed hydrogen and oxygen on GaAs surfaces are calculated. Specific attention is focused on the vibrational energies of surface species and their reported variation with surface stoichiometry. The symmetry and localization of the vibrational states are also discussed. Comparisons are made with experimental high resolution electron energy loss spectroscopy spectra. Our results here agree well with a variety of experimental data. ©1997 American Institute of Physics.