This work reports the synthesis of nanostructured Ce_1-xNi_xO_2-δ (x = 0.05, 0.1, 0.15 and 0.2) oxides prepared by a cation complexation route and with the main objective of studying their redox properties using a combination of electron microscopy, synchrotron radiation X-ray diffraction (SR-XRD) and X-ray absorption near-edge spectroscopy (XANES). The Ce_1-xNi_xO_2-δ series of nanopowders maintain the cubic crystal structure (Fm3m space group) of pure ceria, with an average crystallite size of 5.7 nm indicated by XRD patterns and confirmed by transmission electron microscopy. In situ SR-XRD and XANES carried out under reducing (5% H₂/He; 5% CO/He) and oxidizing (21% O₂/N₂) atmospheres at temperatures up to 500 °C show a Ni solubility limit close to 15 at% in air at room temperature, decreasing to about 10 at% after exposure to 5% H₂/He atmosphere at 500 °C. At room temperature in air, the effect of Ni on the lattice parameter of Ce_1-xNi_xO_2-δ is negligible, whereas a marked expansion of the lattice is observed at 500 °C in reducing conditions. This is shown by XANES to be correlated with the reduction of up to 25% of Ce⁴⁺ cations to the much larger Ce³⁺, possibly accompanied by the formation of oxygen vacancies. The redox ability of the Ce⁴⁺/Ce³⁺ couple in nanocrystalline Ni-substituted ceria is greatly enhanced in comparison to pure ceria or achieved by using other dopants (e.g. Gd, Tb or Pr), where it is limited to less than 5% of Ce cations.