At 1250°C, the following single-phase regions have been detected in the UO 2UO 3EuO 1.5 region of the ternary uranium-europium-oxygen system: (a) β-U 3O 8: no solubility for EuO 1.5 could be observed. (b) Fluorite phase: The oxygen limiting compositions are (U, Eu)O 2.25 and (U, Eu)O 1.79, respectively; the range of the stoichiometric composition MO 2 is between UO 2 and (U 0.36, Eu 0.64)O 2, a fluorite phase with U(VI) is only obtainable for the substoichiometric region from (U 0.32, Eu 0.68) O 1.98 to (U 0.24, Eu 0.76)O 1.86. (c) Rhombohedral phase: It extends from UO 3 · 6EuO 1.5 on the UO 3EuO 1.5 side of the system to the 0.7UO 2·O·3EuO 1.50.6UO 2·0.4EuO 1.5 line on the UO 2EuO 1.5 side, showing an increased phase width with lowering of the uranium valency. (d) Monoclinic B-EuO 1.5: No solubility for UO 2+ x could be observed. The first lanthanide-actinide perovskite compounds, orthorhombic EuUO 3, and EuNpO 3 could be prepared by different methods; attempts to prepare similar ternary oxides containing Th(IV), Pu(IV), and Am(IV), however, failed. EMF measurements have proven that cubic C-EuO 1.5 is the thermodynamically stable EuO 1.5-modification below 1050 ± 20°C.