The surface structure of the clean Co{101̄0} surface and a c(2 × 2) potassium overlayer have been determined by quantitative low energy electron diffraction. The Co{101̄0} sample has been shown to be laterally unreconstructed with the surface being uniquely terminated by an outermost closely packed double layer (dz12 = 0.68 Å). A damped oscillatory relaxation of the outermost three atomic layers occurs, with relaxations Δ dz12 = − 6.5 ± 2% and Δ dz23 = +1.0 ± 2%.The c(2 × 2) overlayer formed at a coverage of 0.5 ML was subjected to a full I-V analysis. A range of adsorption sites were tested including fourfold hollow, on-top, and both long and short bridge sites in combination with both “long” and “short” cobalt interlayer terminations. A clear preference was found for adsorption in the maximal coordination fourfold hollow site. No switching of surface termination occurs. The potassium adatoms reside in the [12̄10] surface channels directly above second layer cobalt atoms with a potassium to outermost cobalt interlayer separation of 2.44 ± 0.05 Å. Potassium-cobalt bond lengths of 3.40 ± 0.05 and 3.12 ± 0.05 Å between the four (one) outermost (second) layer nearest-neighbour substrate atoms suggests a potassium effective radius of 1.87 ± 0.05 Å, somewhat smaller than the Pauling covalent radius and considerably larger than the ionic radius (1.38 Å). The alkali-surface bonding is thus predominantly “covalent”/“metallic”.