Abstract. CAC12.6H20 and SrC12.6H20, isotypic, trigonal, P321, Z= 1, unit cells from X-ray CuKa 1, 2 x= 1.54051, 2N= 1"077]~, room temperature. CaCI2.6H20, M r = 219.08, a = 7.8759 (2), c = 3.9545 (2),/~, V= 212.43 (6),/k 3, D m= 1.71, Dx= 1.7126 gcm -3, g = 2.566 cm -~, F(000) = 14.00 fm, R(F) --0.0287 for 304 unique reflections. SrCI~.6H20, M r=266.62, a=7.9596(3), c=4.1243(2) A, V= 226.29 A 3, D m = 1.93, D x = 1.9565 g cm -3, g = 2.430 cm -1, F(000) = 16.12 fm, R(F) = 0.0356 for 325 unique reflections. Nine water molecules are coordinated to each metal ion. Three of these adjoin only one cation and six are shared between two such ions. These water molecules are hydrogen-bonded to the CI-ions so that each anion is surrounded by six H atoms in a distorted octahedral arrangement.Introduction. These single-crystal neutron diffraction studies are part of a program to locate the H atoms precisely and to investigate the role of water in simple salt hydrates.