The structural and chemical characterization of inverted bi-magnetic MnO x(antiferromagnetic)/MnO y(ferrimagnetic) and FeO x(soft-ferrimagnetic)/MnO x(hard-ferrimagnetic) core/shell nanoparticles has been carried out by means of scanning transmission electron microscopy with electron energy loss spectroscopy analysis, (S)TEM-EELS. Quantitative EELS was applied to assess the local composition of the nanoparticles by evaluating the local Mn oxidation state based on the Mn L 3/L 2 peak intensity ratio and the Mn L 3 peak onset. The analysis allows to unambiguously distinguish the core from the shell and to determine the nature of the involved manganese oxides in both cases. The results evidence that the structure of the nanoparticles is, in fact, more complex than the one designed by the synthesis parameters. © 2011 Elsevier Ltd.