The interaction of oxygen with alkalis (Na, K) on Ni(111) was studied by high-resolution electron energy loss spectroscopy. Loss measurements revealed for the first time a softening of the O–Ni bond and, simultaneously, a strengthening of the alkali-Ni bond in the alkali+O coadsorbed phase, in perfect agreement with recent theoretical calculations. The weakening of the O–Ni bond was ascribed to the alkali-induced filling of the O 2pz antibonding orbitals. Different physical mechanisms were discussed for explaining the strengthening of the alkali-substrate bond whenever alkalis are coadsorbed with O adatoms.