The role of charge transfer effects in the X-ray photoelectron spectra of oxides has been studied using a newly developed ab initio model Hamiltonian description for MnO and NiO. The reason that these effects are a minor perturbation for MnO while they are extremely important for NiO is identified as an atomic property of the transition metal cations. This property is the energetic advantage of adding a d electron to the cations which increases rapidly for heavier metal atoms.
