The structure of (CeRb2)-Rb-III(NO3)(5) . 4H(2)O was determined from single crystal diffraction. The symmetry is monoclinic (space group Cc) : a = 11.050(1) Angstrom, b = 8.977(1) Angstrom, c = 17.859(2) Angstrom beta = 100.877(9)degrees, Z = 4. The structure consists of icosahedra [Ce(NO3)(5)(H2O)(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 CeRb2(NO3)(5) . 3H(2)O, CeRb2(NO3)(5) . 2H(2)O alpha- or beta-CeRb2(NO3)(5) in crystalline or amorphous forms, and the mixture of Ce2Rb3(NO3)(9) and CeRb(NO3)(4), have been shown during the thermal decomposition. The transformations are atmosphere and particle-size dependent, and the dehydration mechanisms are discussed.