Materials based on cerium oxide, stabilized by oxides of rare earth elements, are promising for use in medicine, energy and mechanical engineering due to the uniqueness of their properties. State diagrams of CeO2–La2O3–Ln2O3 systems are the physicochemical basis for the creation of solid electrolytes for fuel cells, oxygen gas sensors, catalyst carriers, protective coatings on alloys, etc. Phase equilibria and structural transformations in CeO2–La2O3–Gd2O3 systems at temperatures 1250 and 1500 °С and in the binary system La2O3–Gd2O3 at temperatures 1100, 1500 and 1600 ° С in the whole range of concentrations were investigated using X-ray phase and microstructural analyzes. It was found that solid solutions based on cubic (F) modification with CeO2 fluorite type, monoclinic (B) and cubic (C) modifications of Gd2O3 and hexagonal (A) modification of La2O3 are formed in the ternary system CeO2–La2O3–Gd2O3. The boundaries of the phase fields and the periods of the crystal lattices of the formed phases are determined. It is established that in the CeO2–La2O3 –Gd2O3 system at 1250 and 1500 °С the phases of cubic symmetry are in equilibrium: on the basis of F–CeО2 with the spatial group Fm3m and C-phase on the basis of Gd2O3 with the spatial group Ia3. As the temperature decreases, there is a narrowing of all areas of homogeneity.