We investigate the local structure of nanoparticles based on a manganese ferrite core surrounded or not by a maghemite layer obtained after hydrothermal surface treatment. Results of X-ray powder diffraction (XRD) and neutron powder diffraction (NPD) measurements are crossed with those of infield Mo??ssbauer spectroscopy and X-ray absorption spectroscopy (XANES/EXAFS) to study the valence state of Mn ions and the cation distribution at interstitial sites of the core???shell nanoparticle structure. Linear combination fitting of XANES data clearly indicates the existence of mixed valence states of Mn cations in the Mn ferrite phase. As a direct consequence, it induces nonequilibrium cation distributions in the nanoparticle core with the presence of a large amount of Mn cations at octahedral sites. The quantitative results of the inversion degree given by NPD, Mo??ssbauer spectroscopy measurements, and EXAFS are in good accordance. It is also shown that both the proportions of each oxidation degree of Mn ions and their location at tetrahedral or octahedral sites of the spinel nanocrystal core can be modified by increasing the duration of the surface treatment.