Chemical preparation and crystal structure are given for a new monovalent-divalent cation cyclohexaphosphate. This compound is monoclinic P2(1)/c with the following unit cell parameters : a = 9.087(2), b = 12.246(4), c = 9.895(4) Angstrom, beta = 116.98(2)degrees, Z = 2, V = 981(1) Angstrom(3), and Dx = 2.897 g . cm(-3). The atomic arrangement can be described as successive wide layers made by the P6O18 ring anions and interconnected by the CaO6 and CsO8 polyhedra and the hydrogen bonds. Crystal structure was solved with a final R value of 0.032 for 6263 independent reflections. The thermal behavior has been investigated and interpreted by comparison with IR absorption spectroscopy and X-ray diffraction experiments.