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The phase relations in a CeO2–Gd2O3–ThO2 system have been established, for the first time, after slowly cooling the samples from 1400°C. Ceria has been used as a surrogate material in place of plutonia. About 70 compositions in Th1−xGdxO2−x/2, Ce1−xGdxO2−x/2, (Th0.5Ce0.5)1−xGdxO2−x/2, (Ce0.5Gd0.5)1−xThxO1.75+x/4, and (Th0.5Gd0.5)1−xCexO1.75+x/4 systems were prepared. X-ray diffraction data revealed the presence of different phase regions, namely cubic fluorite-type solid solution, C-type solid solution, and biphasic regions containing C- and F-type solid solutions, and two F-type solid solutions could be delineated. This ternary system showed the existence of a wide cubic phase field. ThO2 was found to dissolve up to about 40 mol% GdO1.5. The GdxTh1−xO2−x/2 system has a biphasic region consisting of C- and F-type solid solutions, whereas GdxCe1−xO2−x/2 system supported only a single phase (F or C) throughout. A striking observation was that the anion-excess gadolinia, i.e., Gd1−xCexO1.5+x/2, was found to retain the C-type lattice, even after heat treatment, unlike pure gadolinia.