Composite nanoparticles containing a γ-Fe₂O₃ core, Ni₂O₃ external shell and FeCl₃·6H₂O outermost layer can be synthesized by chemically induced transition in FeCl₂ solution. These may be modified by treatment with Fe(NO₃)₃ to obtain particles for the preparation of ionic ferrofluids. Vibrating sample magnetometer (VSM) measurements and transmission electron microscopy (TEM) observations show that after Fe(NO₃)₃ treatment, the specific magnetization becomes weaker and the size becomes larger for treated particles compared with the untreated particles. Using energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), the structure of the particles before and after the treatment is revealed. The experimental results show that the γ-Fe₂O₃ core is unchanged, the Ni₂O₃ is dissolved partially and the FeCl₃·6H₂O is replaced by Fe(NO₃)₃·9H₂O. The percentages of molar, mass and volume of these phases are deduced, and the average density of the modified particles is also estimated.