We discuss the nature and energy of the affinity levels of molecules on metals, on the basis of recent inverse photoemission experiments and ab-initio cluster calculations. We show that the energy and composition of the affinity levels such as the 2π* level of CO are not determined by the interaction of occupied d-band states with the empty adsorbate levels, but that hybridization with nearresonant unoccupied metal states is important. The nature of the bonding of the negative ions produced by the population of the affinity levels in inverse photoemission spectroscopy (IPES), and possible screening mechanisms in IPES are examined. We then discuss the electronic excitations of the adsorbales. The role of charge transfer in determining the bonding energies of ground state and excited state molecules to the metal is examined using ab-initio cluster calculations on chemisorbed CO and NO as examples. It is shown that while large charge transfer may lead to strong chemisorption in the ground state, it may destabilize the excited state and lead to desorption or fragmentation. The mechanism by which valence ionization of chemisorbed CO may lead to CO+ desorption is also analysed. Finally, we compare IPES and excitation spectroscopy as methods for the study of affinity levels. © 1986.