Abstract: The phase composition, the structure of cores and shells, and the dependences of the shell thickness on the fabrication technique were determined by Mössbauer spectroscopy for core/shell Fe₃O₄/γ-Fe₂O₃ magnetic nanoparticles (MNPs) with Fe₃O₄ cores of a fixed size (8 nm) and γ-Fe₂O₃ shells of a varying thickness (1, 3, or 5 nm). A layer on the MNP surface with a magnetic state differing from that in the bulk was found. It was also found that a spin-glass state probably exists between the core and the shell. The studied core/shell Fe₃O₄/γ-Fe₂O₃ MNPs have promising applications in various fields (e.g., biomedicine).