Crystal data and atomic arrangement are given for a new rare-earth cyclohexaphosphate decahydrate : Er2P6O18.10H(2)O. This compound is monoclinic P2(1)/a, with a = 18.315(8), b = 13.343(4), c = 11.211(4)Angstrom, b = 94.46(3)degrees, Z = 4, V = 2681(3) Angstrom(3). The crystal structure was determined with a final R = 0.032 for 4897 independent reflections. It can be described as a succession of thick corrugated layers of alternated P6O18 rings and ErO8 polyhedra with wide channels where non-bonded water molecules are located. The phosphoric ring anion has no internal symmetry. The erbium polyhedra do not share any edge or corner.