The coadsorption of Na and CO on Ni(111) at room temperature was investigated using high-resolution electron energy loss spectroscopy. A notable softening of the Na–Ni bond was revealed in the Na+CO coadsorption phase. A lengthening of the Na–Ni bond was observed upon annealing the substrate. Oxygen adsorption on Na+CO/Ni(111) caused a significant blue-shift of the C–O stretching frequency. Exposing the Na/Ni(111) to oxygen, no Na–O bond was observed. On the contrary, the formation of the Na–O bond was easily achieved by exposing the Na+CO/Ni(111) phase to oxygen.