High-resolution electron energy loss spectroscopy was used to investigate alkali (Na, K) coadsorption with CO and O on Cu (111) and Ni (111). Measurements provided new insights in these systems. A CO-induced weakening of the alkali–substrate bond was revealed on both substrates. The effect is more pronounced for the Na+ CO/Ni (111) system. Submonolayers of alkalis were found to promote the preferential population of the subsurface site for O/Cu (111) but not for O/Ni (111).