The asymmetric unit of the title salt, 2NH4+_B10H102__1.5H2O or (NH4)2B10H10_1.5H2O, (I), contains two B10H102_ anions, four NH4+ cations and three water molecules. (I) was converted to the anhydrous compound (NH4)2B10H10, (II), by heating to 343 K and its X-ray powder pattern was obtained. The extended structure of (I) shows two types of hydrogen-bonding interactions (N-H_ _ _O and O-H_ _ _O) and two types of dihydrogen-bonding interactions (N- H_ _ _H-B and O-H_ _ _H-B). The N-H_ _ _H-B dihydrogen bonding forms a two-dimensional sheet structure, and hydrogen bonding (N-H_ _ _O and O-H_ _ _O) and O- H_ _ _H-B dihydrogen bonding link the respective sheets to form a three-dimensional polymeric network structure. Compound (II) has been shown to form a polymer with the accompanying loss of H2 at a faster rate than (NH4)2B12H12 and we believe that this is due to the stronger dihydrogenbonding interactions shown in the hydrate (I).