The chemical preparation and the main crystallographic, calorimetric, and structural features are given for a new alkyldiammonium antimony halogenide. [NH3(CH2)(2)NH3](5)(Sb2Br11)(2) . 4H(2)O salt is monoclinic P2(1)/a with unit-cell dimensions a = 14.346(2), b = 13.206(1), c = 17.027(2) Angstrom, beta = 113.05(1)degrees, Z = 2; D-m = 2.985 M g(-3); D-x = 2.941 M g(-3); mu = 127.3 cm(-1); F(000) = 2388; T = 298 K; R = 0.052 and R-w = 0.057 for 4136 observed reflections. The examination of the structure shows a layer arrangement parallel to the b axis : planes of (Sb2Br11)(5-) bioctahedra alternate with planes of [(CH2)(2)(NH3)(2)](2+). The (Sb2Br11)(5-) bioctahedra are connected through O(W)-H...Br hydrogen bonds, so that infinite chains of the composition [Sb2Br11(H2O)](n)(5n-) are formed in the structure parallel to the b axis. The structure of this compound is built from (Sb2Br11)(5-) anions, [(CH2)(2)(NH3)(2)](2+) cations, and zeolitic water molecules connected by hydrogen bonds. The thermogravimetric analysis and differential scanning calorimetric studies show that the dehydration of this salt occurs in two steps : at 377 and 391 K, respectively, with a loss of two water molecules per step. The anhydrous salt starts to decompose at 490 K.