The genesis and structural properties of (Ce1–xMx)0.8Ni0.2Oy (M = La, Mg; x = 0, 0.2, 0.5, 0.8, 1; 1.0 ≤ y ≤ 1.8) oxides that are promising catalyst precursors are studied. A complex of physicochemical methods (low-temperature nitrogen adsorption, ex situ and in situ powder X-ray diffraction, electron microscopy, Raman spectroscopy, and EDX analysis) is used to show that after the calcination at 500 °C in the oxidizing medium, (Ce1–xMx)0.8Ni0.2Oy become mesoporous materials representing substitutional solid solutions with a cubic fluorite-type structure with supported particles of the Ni-containing phase. When M = La, Ni cations are mainly stabilized in the composition of the Ce–La–Ni–O solid solution, and when M = Mg, they are stabilized in the Mg–Ni–O composition. It is found that La- or Mg-containing samples, as compared to Ce0.8Ni0.2Oy, have highly defective structures and smaller sizes of crystallites (≤ 4 nm, x ≥ 0.5). After thermal treatment of the samples in H2/Не at 800 °C, highly dispersed Ni° particles with a size of 5-10 nm are formed. Metal particles have defects and a polycrystal structure, and they are readily reoxidized in air with the formation of “core@shell” Ni@NiO structures.