The Au(110) surface is reconstructed and shows a (1×2) low-energy electron-diffraction (LEED) pattern, which is known to correspond to a missing-row reconstruction. At a coverage of half a monolayer of potassium, a c(2×2) LEED pattern is seen, which is not easily rationalized in terms of the faceted structures observed at lower alkali-metal coverages. Two different models have been proposed for the atomic arrangements in this structure. We have performed a medium-energy ion-scattering study, using channeling and blocking, of this surface. We find very good agreement with a model first proposed by Ho et al.: The top layer of atoms consists of intersecting c(2×2) arrays of Au and K atoms. The top layer of Au atoms is shifted toward the bulk, and the K atoms are displaced outward relative to this plane. The third layer of Au atoms is also buckled, all in quantitative agreement with the calculation. © 1989 The American Physical Society.