M, = 883.28, monoclinic, P21/c, a = 7.279 (2), b= 13.984 (8), c= 17.071 (4)/k, fl= 90.42 (2) °, V= 1738 (2) A 3, Z - 4, D m not measured, D x = 3.376 Mg m -a, 2(Ag Ka) = 0.5608 A, It = 4.33 mm -1, F(000) = 1592, T= 293 K, final R value 0.024 for 4512 independent observed reflexions. Planes of Te(OH) 6 groups at z- 0 and 0.5 alternate with planes of P309 ring anions at z = 0.25 and 0.75. The H atoms were located by difference-Fourier synthesis and their positions have been refined. A complete scheme of the hydrogen bonds is given. Introduction. A few adducts of telluric acid with cyclo-triphosphates of monovalent cations have already been investigated: Te(OH)6.2Na3PaO9.6H20 (Boud- jada, Averbuch-Pouchot & Durif, 1981a), Te(OH)6.- K3P3Og.2H20 (Boudjada, Averbuch-Pouchot & Durif, 1981b), Te(OH)6.Rb3P3Og.H20 (Boudjada & Durif, 1982) and Te(OH)6.2(NH4)3P30 9 (Boudjada, Boudjada & Guitel, 1983). Up to now, all attempts to prepare a similar caesium adduct have been unsuccessful because of the difficulty in preparing large amounts of caesium cyclo-triphos- phate. This starting material is normally prepared by Boull6's (1941) metathesis reaction according to the