INDO/CI calculations were used to analyze the C1s and O1s shake-up spectra of nickel tetracarbonyl, Ni(CO)4. The satellite structure in both cases is dominated by excitations from metal–ligand bonding (2Πb) to metal–ligand antibonding (2Πa) orbitals and by excitations within the core-ionized CO molecule, ΠCO—Π*CO.