We have measured the photoemission spectra both for CO adsorbed on Ni and Pd films and for gas phase CO over a wide photon energy range. The comparison of these two sets of measurements lead to the following interpreation of the two observed electronic energy levels of molecularly adsorbed CO. The lower binding energy CO-induced level corresponds to the 1π gas phase level with some admixture of the 5σ level and the higher binding energy level corresponds to the 4σ gas phase level. The hω-dependence of the photoionization cross section is shown to play a predominant role in determining the surface sensitivity. © 1975.