X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and vibrating sample magnetometry (VSM) were used to investigate the influence of Ni ions on the structural, electronic, and magnetic properties of nickelferrites (Ni_xFe_3-xO₄). Spinel Ni_xFe_3-xO₄ (x ≤ 0.96) samples were prepared as polycrystalline thin films on Al₂O₃ (0001) substrates, using a sol-gel method. XRD patterns of the nickel-ferrites indicate that as the Ni composition increases (x > 0.3), a structural phase transition takes place from cubic to tetragonal lattice. The XPS results imply that the Ni ions in Ni_xFe_3-xO₄ substitute for the octahedral sites of the spinel lattice, mostly with the ionic valence of +2. The minority-spin d-electrons of the Ni²⁺ ions are mainly distributed below the Fermi level (E_F), at around 3 eV; while those of the Fe²⁺ ions are distributed closer to E_F (~1 eV below E_F). The magnetic hysteresis curves of the Ni_xFe_3-xO₄ films measured by VSM show that as x increases, the saturation magnetization (M_s) linearly decreases. The decreasing trend is primarily attributable to the decrease in net spin magnetic moment, by the Ni²⁺ (2 μB) substitution for octahedral Fe²⁺ (4 μB) site.