Chemical effects are observed in the M4,5NN Auger spectrum of Mo following adsorption of oxygen and carbon monoxide on clean Mo(110). This system is particularly interesting as Auger transitions involving only core electrons, one conduction band electron or two conduction band electrons are all present over the energy range 90−250 eV. On adsorption, Auger transitions involving only core electrons shift to lower energies, whereas those involving one or two conduction band electrons undergo changes in shape. Auger spectra are presented in both derivative form 𝒩(1)m(E) and following integration, 𝒩m(E).