Abstract The analysis of core-level and valence band photoemission spectra of NiO and CuO, yields that the final states of the former should be assigned to cd9 L (main peak, lower binding energy) and cd8 (satellite) states, both configurations being considerably intermixed by covalency. On the basis of cluster calculations it is shown that non-local screening effects with contributions from dn+1dn−1 charge transfer states and multiplet structure have a distinct influence on the energies of the core-level photoemission lines and should be discussed on an equal footing when considering photoemission data of solids built up of transition metals and bridging ligands. Ab initio calculations based on isolated MO6 units yield rather high values of the ligand-to-metal charge transfer energy Δ, compared to the experimental value of solids such as NiO. This is most probably due to the neglect of electronic delocalization as the consequence of the charge transfer to neighbouring metal and more distant ligand atoms and to the additional relaxation for excited charge transfer states with contributions from the remote surrounding. A correlation between non-local screening and magnetic coupling is pointed out, and their effect on core-level photoemission line shapes is explored theoretically and supported experimentally by available temperature and concentration dependent 2p XPS spectra of NixMg1−xO mixed crystals. UV absorption spectra of the NixMg1−xO mixed crystals series are reported and show large red shifts with increasing x, favouring a charge transfer mechanism with non-local contributions from neighbouring metals and more distant ligands. The energies of spin-allowed and intensities of spin-forbidden transitions in the d–d spectra reflect, in an attenuated way, the large energetic effects in the excited states when going from isolated to interconnected polyhedra, as manifested in a lowering of the Racah parameter B with increasing x. A further red shift of the charge transfer bands is observed when hole doping in NiO occurs (Ni1−δO, δ>0). This result can be accounted for by extending the presented model to oxidic systems with hole doping. EPR experiments show that Ni(III) in oxide ceramics possesses a low-spin d7 ground state in isolated NiO6 centres (large Δ value), while a d8 L ground state may be present in the case of interconnected polyhedra with extended electron delocalization (smaller Δ value).