Using a point charge model, the electrostatic contribution to the core level electron binding energies in ionic adsorbate layers is examined. The system considered as an illustrative example is K and CO coadsorbed on transition metal surfaces. In an ionic model for coadsorption, the shifts in the K, C, and O core levels with respect to the singly adsorbed systems have two dominant contributions: a chemical shift due to charge transfer from K to CO and an offsetting shift due to a Madelung term deriving from Coulombic interaction of the core hole with the ionic surface layer. The Madelung potential is evaluated explicitly while the change in screening due to the addition or removal of a valence electron upon the formation of a core hole is obtained via an “equivalent core” approximation. Taken together, the two terms cause a shift to lower binding energy of all adsorbate core levels, offering a potential resolution to an apparent paradox for this coadsorbed system. For adsorbate surface densities typically observed for K + CO, it is found that the Madelung shift would be large, of order 7–8 eV, and must be considered to understand the photoemission results.