We investigate the electronic structure of cobalt-nickel mixed oxides by means of SR-based electron spectroscopy. We vary the Co-Ni content systematically and find that the most significant changes in the spectroscopic data occur at a mixing ratio of 30/70. We analyze the electronic structure of this material in detail and we are able to distinguish between pure covalent contributions, which cause a band gap of about 6 eV and empty defect states, which fill the band gap almost completely. These defects are associated with the formation of 3d(7)L charge transfer states. These are correlated defect states and are identified by resonant photoemission spectroscopy. (C) 2012 Elsevier B.V. All rights reserved.