Linear NiCO clusters are used to model the chemisorptive interaction of CO with the Ni surface. The electronic structures of initial states as well as fully relaxed final CO core hole states of the cluster are obtained from ab initio Hartree-Fock LCAO calculations. For both O 1s and C 1s ionization two relaxed final states are found corresponding to the screened and unscreened core hole states in the Schönhammer-Gunnarsson model. For a given core hole the difference between the two ionization potentials is only weakly dependent on the choice of the NiCO distance, dNiCO, in the cluster whereas the intensity ratio of the two final hole states, computed in the sudden approximation, varies strongly with dNiCO. XPS experiments for both O 1s and C 1s ionization of CO adsorbed on nickel show a two peak structure. The energy separation and intensity ratio of the two peaks are explained by the present cluster model for reasonable dNiCO values. © 1979.