Antimony overlayers on the GaAs(110) surface have been studied by scanning tunneling microscopy and spectroscopy. The Sb is observed to grow as a 1×1 ordered monolayer. The positions of the Sb atoms in the surface unit cell are deduced from voltage-dependent imaging, and compared with previously proposed structural models. A best fit is found for a model in which the Sb atoms occupy positions similar to what would be the positions of Ga and As atoms at an unrelaxed GaAs(110) surface, in agreement with the results from low-energy electron-diffraction experiments. The surface-state density is measured with tunneling spectroscopy. Two filled-state peaks and one empty-state peak are observed in the spectra, as well as a band-gap region with a width nearly equal to the bulk band gap. These observations are compared with previous photoemission results and theoretical calculations. © 1989 The American Physical Society.