Polycrystalline Ni_xFe_3-xO₄ thin films prepared by using a sol-gel process exhibited phase-pure spinel (Fd3m) structure for the Ni composition up to x = 1.0. X-ray photoemission spectroscopy (XPS) and Raman spectroscopy investigations revealed that Ni ions mostly have valence of +2 and occupy the octahedral sites of the cationic sublattice. The Ni_xFe_3-xO₄ films exhibited higher electrical resistivity than that of Fe₃O₄ below 300 K. The increase in the resistivity is primarily ascribed to decrease in Fe²⁺ -Fe³⁺ polaronic hopping rate in the octahedral sites due to the increase in the octahedral Ni²⁺ population.