Orbital energy spectra of CO and Hg adsorbed individually and co-adsorbed on Ni (100) have been determined by ion-neutralization spectroscopy (INS) and ultraviolet photoelectron spectroscopy (UPS) at 45 ° incidence. In the case of CO, two orbitals derived from the 5σ and 1π orbitals of free CO are observed 7.8 and 11.1 eV below the Fermi level, respectively, by UPS, with INS revealing only the 7.8-eV orbital since 11.1 eV is outside its accessible range. In the case of Hg, UPS and INS both reveal orbital peaks 7.8 and 9.7 eV belowEF, identified, respectively, with the5d5/2and5d3/2states of Hg. The Hg(6s) orbital is not in evidence in the energy spectra obtained by either method. We have determined energy shifts of the observed orbitals with respect to their positions in the free molecule or atom and have discussed these shifts in terms of the effects of bonding and electric charge shifts in the metal-adsorbate complexes. This work permits a fairly detailed intercomparison of INS and UPS. It also bears on the problem of peak shifts due to interference between bulk and surface photoemission and possible perturbation by the probing ion in the case of ion neutralization.