Electronic excitations of sodium and of sodium coadsorbed with oxygen on Ru(001) have been investigated by electron energy loss spectroscopy in N(E) and d 2 n/d E 2 mode. For sodium adsorbed on Ru(001) a characteristic, sharp low-energy loss feature at 3–4 eV loss energy is observed, which shifts in energy as a function of Na coverage. This loss is discussed in terms of a one-electron excitation involving a Na(3s)−Ru(4d) interface state at low Na coverages, but is interpreted as a plasmon loss of the Na layer as the Na coverage approaches one monolayer. Both one-electron transition and plasmon excitation are strongly influenced and quenched by coadsorption of oxygen. The disappearance of the one-electron excitation is interpreted in terms of a modification of the interface state by coadsorbed oxygen, and the destruction of the Na plasmon resonance is associated with the loss of free-electron character in the Na layer as a result of oxidation.