Possessing high H2 capacities and interesting dehydrogenation behavior, metal amidoborane ammoniates were prepared by reacting Ca(NH2)2, MgNH, and LiNH2 with ammonia borane to form Ca(NH2BH3)2⋅2 NH3, Mg(NH2BH3)2⋅NH3, and Li(NH2BH3)2⋅NH3 (LiAB⋅NH3). Insight into the mechanisms of amidoborane ammoniate formation and dehydrogenation was obtained by using isotopic labeling techniques. Selective 15N and 2H labeling showed that the formation of the ammoniate occurs via the transfer of one H(N) from ammonia borane to the [NH2]− unit in Ca(NH2)2 giving rise to NH3 and [NH2BH3]−. Supported by theoretical calculations, it is suggested that the improved dehydrogenation properties of metal amidoborane ammoniates compared to metal amidoboranes are a result of the participation of a strong dihydrogen bond between the NH3 molecule and [NH2BH3]−. Our study elucidates the reaction pathway involved in the synthesis and dehydrogenation of Ca(NH2BH3)2⋅2 NH3, and clarifies our understanding of the role of NH3, that is, it is not only involved in stabilizing the structure, but also in improving the dehydrogenation properties of metal amidoboranes.