The structures of halogen monolayers (I, Br, Cl, and F) and mixed-halogen monolayers on Ag(111) surfaces are measured by scanning tunneling microscopy, (STM). Adatom radii determined from nearest-neighbor bond distances are in excellent agreement with literature values of the radii of halides, indicating that the adsorbed halogen atoms have an essentially ionic character and are adsorbed as a surface Ag-halide complex. STM images of mixed-halogen adlayers, comprised of either coadsorbed I and Cl, I and F, or Cl and F, exhibit several structural features, e.g., a low-frequency moire pattern and a large atom vacancy density, that are not observed in single-atom adlayers and that are ascribed to Coulombic forces within the ionic adlayer. STM has also been used to monitor variations in adlayer structures following ion exchange of chemically distinct halogen species. STM images and data obtained from Auger electron spectroscopy indicate that the chemical stability of the surface Ag-halide complexes in aqueous solutions decreases with increasing halogen electronegativity. The results suggest that the energy of solvation of the halide ions determines the relative stability of the Ag-halide surface bond, similar to that observed for bulk silver halide crystals.