In the chemisorption system CO/Ni(100), a comparison between available high resolution core level photoelectron spectra and spectra calculated by the INDO/CI method done for different nickel cluster sizes. A reassignment of some of the satellites is made. It is found that the asymmetry and the broadening of the main line mainly depends on substrate-substrate excitations, predominantly from the d-band to the s-band within the nickelsubstrate.