The interaction between ammonium dinitramide (ADN) and water was experimentally investigated by determining the solid-liquid phase diagram for the system ADN-water and by measuring its critical relative humidity at 25 degrees C. Additionally, the density of aqueous ADN solutions of different compositions were measured in the temperature range from (5 to 85)degrees C. The properties of ADN were compared to ammonium nitrate (AN), and the results show that ADN is more hygroscopic than AN. The density data show that aqueous ADN solutions behave as an ideal solution, with a partial molar volume independent of the composition. This made it possible to determine the molar volume and the density of pure ADN in the liquid state as well as the volumetric contraction of molten ADN on solidification and cooling.