9 pags., 7 figs., 1 tab. (111)-oriented NiFeO thin films with varying Ni content (0.7 ≤ x ≤ 1.7) have been grown on c-plane (0001) α-AlO substrates by ion beam sputtering. Epitaxial NiFeO thin films with spinel structure are obtained for x ≤ 1. For nickel content exceeding the value of the stoichiometric phase, i.e. NiFeO, a transition from an ordered spinel to a disordered rock-salt phase is identified, being the dominant one when x = 1.7. This result is confirmed by X-ray diffraction, Rutherford backscattering spectroscopy in channeling configuration, Raman spectroscopy, X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). We also report the first Raman spectra in NiFeO with mainly rock-salt structure. The evolution of the electrical and optical properties of the (111) NiFeO films as Ni content increases, supports the structural change. The electrical resistivity increases up to 10 ohm·cm, the direct and indirect band gaps also increase and only one direct band gap, instead of the two characteristic of the spinel structure, is present in the NiFeO film with x = 1.7. This investigation has been funded by the Comunidad de Madrid through the NANOMAGCOSTCM-P2018/NMT4321 project and by the MINECO of Spain through the RTI2018–095303-A-C52 and RTI2018–095303-A-C51 projects. We acknowledge the support from CMAM for the beam time proposals with codes P00736 and P00684. Part of these experiments were performed at the CLÆSS-BL22 beamline at ALBA Synchrotron (grant number 2020094658). Giulio Gorni is especially grateful for his support during the XAS mea- surements. A.S. acknowledges the financial support from the Comunidad de Madrid for an “Atracción de Talento Investigador” contract (No. 2017-t2/IND5395). We also acknowledge the support of Miguel Manso Silván for the optical characterization of the films. The financial support from the Estonian Research Council grants PSG-466 is gratefully acknowledged