The in situ formation mechanism of amides by ammonolysis of the lightest alkali and alkalineearth metal hydrides is investigated. Novel information, particularly on the ammonolysis process of KH is given in detail. Furthermore, the hydrogen sorption properties, reaction mechanisms and phase evolution of the K-Mg-N-H system are studied in situ. The desorption temperature and reaction pathways of the composite systems KNH2 +MgH2, KH+Mg(NH2)2 are analysed and compared underlining differences in their thermodynamic and kinetic properties. As a consequence, the KNH2-KH composite system is systematically explored in the entire compositional range, in order to isolate and characterize the novel intermediates evidenced during the ammonolysis of KH and during the desorption process of K–Mg–N–H based systems. The investigation is enlarged to include the Rb-N-H and Rb-Mg-N-H systems, providing novel structural analogies which are discussed in detail. The insights presented in this work expand the knowledge about the crystal chemistry of metal amides, thus helping to improve the understanding of the role of K- and Rb-based additives on the hydrogen desorption/absorption properties of the Li-Mg-N-H system.