We studied the Ni 2p core-level spectra of NiO using cluster model calculations and X-ray photoemission spectra. The calculations were performed for a NiO6, a NiO5, and a NiO6:NiO5 cluster, which allows studying the bulk, the surface, and the non-local effects in the spectra. The photoemission spectra were taken with soft (or hard) X-rays to enhance the surface (or bulk) contribution. The calculation shows that both surface and non-local-effects contribute to the shoulder in the experimental data. The photoemission spectra show that the relative intensity of the surface contribution decreases for Hard X-rays. (C) 2010 Elsevier B. V. All rights reserved.