The proposed experiment at ISIS aims at the characterization of the vibrational properties of ammonium borohydride, NH4BH4, which possess the highest gravimetric H density (~25 wt.%) of any chemical compound. The H disordered crystal structure is composed of the tetrahedral complex ions NH4+ and BH4-. Recent investigations revealed the presence of dynamic disorder, corresponding to very rapid reorientation of both ions, in the temperature range 100 – 250 K. The elucidation of the vibrational properties of NH4BH4 from INS spectroscopy will give new fundamental understanding of dihydrogen bonding in boron-nitrogen based hydrides. It particular this concerns successive ordering processes which are expected to take place below 100 K. These should reveal from the temperature dependence of the internal modes and the possible presence of librational modes of the tetrahedral complexes.