Ammonium dinitramide (ADN), NH(4)N(NO(2))(2), is one of the most promising oxidizing components of future solid propellant formulations, because it is eco-friendly and more energetic and has no plume signature because there is no chlorine in its molecular structure. The hygroscopicity of the ADN crystal, which is more severe than that of ammonium perchlorate (AP), seriously affects its use. This quite different behavior is possibly associated with their structures. The X-ray single-crystal diffraction data show that vast hydrogen bonds link the ADN structure. The three-dimensional structure of ADN joined by the fourth longer hydrogen bond is an unusual two-fold three-dimensional interpropagation network structure, and the hydrogen bonds in the ADN crystal are much shorter than that in AP because of the special structure. The hydrogen bonds between ADN and water are assayed by IR spectroscopy. Differential scanning calorimetric (DSC) analysis of moisture-containing ADN and AP crystals also confirm that, because of the stronger hydrogen bond between ADN and water molecules, large amounts of bound water are present in the ADN besides some unbound water. All of this evidence suggests that the special hydrogen bonds among ADN and stronger hydrogen bonds between ADN and water molecules could possibly be the main reason of the severe hygroscopicity of ADN.