NiFe2O4 nanocrystalline samples were synthesized by the sol-gel method. The X-ray diffraction patterns of the as-synthesized samples (S- NiFe2O4) showed the formation of main spinel ferrite structure, with an average crystalline size of ∼50 nm. After a washing process with magnetic separation two sets of NiFe2O4 particles was observed, one with ∼36 nm and other with very small size (∼2–3 nm). X-ray photoelectron spectroscopy (XPS) indicated the presence of Fe3+ and Ni2+ ions on the sample surface. Mössbauer spectra were recorded at 77 K and room temperature and were least square fitted for two sextets which have been assigned to the iron ions occupying the two symmetry sites of the ferrite structure. Magnetic measurements are consistent with the magnetic moment of Ni2+ ions suggesting the formation of an ideal inverse spinel structure in the sample S-NiFe2O4. Moreover, after the washing process with magnetic separation (sample P- NiFe2O4), when the formation of extremely small particles was determined, the Mössbauer and magnetic measurements indicated the reorganization of Ni2+/Fe3+ ions in the tetragonal and octahedral sites of the spinel structure and some features related to thermal relaxations of the small particles are determined.