Cluster-size-dependent binding energy (BE) shifts of Ni 2p(3/2) spectra in Ni clusters with respect to bulk Ni metal have been studied as a function of Ni coverage on clean rutile TiO2(0 0 1) surfaces at room temperature. Auger parameter (AP) analysis of photoelectron spectra has been employed and revealed an obvious initial state contribution at the coverage of 0.5 monolayers (ML). The initial state effect was demonstrated to be strongly affected by the substrate and was assigned to a combination of eigenvalue shift in surface core-level shift (SCLS) and charge transfer between the metal clusters and substrates. The TiO2(0 0 1) surface stoichiometry was found to introduce different charge transfer behaviors. Our results experimentally present that the Ni clusters are charged positively on stoichiomtric TiO2 surface and less positively or even negatively on various reduced surfaces. (C) 2009 Elsevier B.V. All rights reserved.