We discuss the factors which determine the energy and composition of the affinity levels of adsorbates on metals. Chemisorbed CO is used throughout as a prototype. We find that the affinity level of chemisorbed CO has significant metal character and thus, image-like relaxation shifts should not be large. Moreover, although we find that, under certain conditions, an adsorbate → substrate charge transfer screening mechanism can operate in inverse photoemission, this mechanism is not likely to operate in the case of CO. We conclude that the position of the affinity level is primarily determined by the downward energy shift induced by the metal effective potential. We indicate that in the excited states of molecular adsorbates, such as CO, the electron-hole interaction, U, is incompletely screened. As a result the affinity level probed by electronic excitations is not identical to that probed by inverse photoemission. The nature of the changes induced by U are discussed. Finally, we briefly comment on the relation between recent spectroscopic findings and theoretical descriptions of vibrational relaxation. © IOP Publishing Ltd.