The stability and decomposition of CaUO 4, Ca 2UO 5, and Ca 3UO 6 on heating in hydrogen were investigated by X-ray powder diffraction and thermogravimetry. Ca 2UO 5 decomposes at 450°C into Ca 2UO 4.5 with a triclinic unit cell. At 850°C, it changes to monoclinic Ca 2(Ca 0.67U 0.33)UO 6 which loses some oxygen up to the composition Ca 2(Ca 0.67U 0.33)UO 5.83. At 1100°C, it decomposs to UO 2 solid solution and CaO. CaUO 4 decomposes at 900°C to Ca 2(Ca 0.67U 0.33)UO 5.83 and CaU 2O 6. The decomposition products of Ca 3UO 6 at 850°C are Ca 2(Ca 0.67U 0.33)UO 5.83 and CaO.