Electron energy loss experiments provide evidence for the two-dimensional spatial localization of the metal/molecule charge-transfer excited state of pyromellitic dianhydride adsorbed on copper single-crystal surfaces in ultrahigh vacuum. Codosing the surface with atomic cesium splits the interfacial loss peak into a doublet, whose energies are independent of cesium coverage. This behavior is in contrast to the smooth, continuous shift in energy expected from the classical picture of alkali metal adsorption in which the substrate Fermi energy increases with coverage at low coverage. Line shape analysis suggests that the localized region includes the four adsorbed molecules nearest each cesium adatom,