The structure of Ce I I I Rb 2 (NO 3 ) 5 4H 2 O was determined from single crystal diffraction. The symmetry is monoclinic (space group Cc): a = 11.050(1) , b = 8.977(1) , c = 17.859(2) , β = 100.877(9)°, Z = 4. The structure consists of icosahedra [Ce(NO 3 ) 5 (H 2 O) 2 ] 2 - linked by hydrogen bonds. The Rb atoms and two water molecules are located between these polyhedra and ranged in an alternating sequence along [010]. From temperature-dependent X-ray diffraction and thermogravimetry measurements, the occurrence of CeRb 2 (NO 3 ) 5 3H 2 O, CeRb 2 (NO 3 ) 5 2H 2 O, α- or β-CeRb 2 (NO 3 ) 5 in crystalline or amorphous forms, and the mixture of Ce 2 Rb 3 (NO 3 ) 9 and CeRb(NO 3 ) 4 , have been shown during the thermal decomposition. The transformations are atmosphere and particle-size dependent, and the dehydration mechanisms are discussed.