LaNiO₃ is one of conductive oxides with a crystal structure suitable for integration in epitaxial heterostructures with perovskites of enormous technological potential such as colossal magnetoresistance materials, high-temperature superconductors and ferroelectrics. The surface segregation of elements and possible hydrooxidation of LaNiO_3-x films were studied by means of Tunable High-Energy X-ray photoelectron spectroscopy using synchrotron radiation. Epitaxial LaNiO_3-x films deposited onto (100)-plane oriented NdGdO₃ substrate was obtained by using a reactive DC magnetron sputtering technique and demonstrate the excellent in-plane orientation and the surface La/Ni ratio close to the bulk stoichiometric value. The films were examined at different angles to distinguish between the chemical state of lanthanum and nickel species at the surface and slightly deeper into the material. The evident difference were observed between spectra measured at normal (Θ = 0°) and grazing (Θ = 89.3°) take-off angles, indicating that the significant alteration of the film chemical composition take place within thin (scanning depth ~ 3 nm) surface layer. Both core and satellite peaks were used for the identification of the chemical state of elements and further quantitative analysis. It was shown, that the hydroxyl-containing phase, located near the film surface may be attributed to the lanthanum and nickel hydroxide species. For the all spectral lines in question hydroxide/oxide peaks intensity ratio strongly affected by the geometry of the experiment, thus directly indicating that the thickness of the hydroxide enriched layer is of about few nanometers. The more quantitative analyses were carried out assuming the exponential and step spatial distributions of hydroxide species concentration within the bulk of the film. The thickness of about 2 nm for hydroxide enriched layer, was estimated from the oxide and hydroxide peak intensities ratio.