Ammonium borohydride, NH4BH4, has the highest gravimetric and volumetric hydrogen density among known inorganic compounds and a fascinating rock salt type crystal structure composed of H disordered tetrahedral complexes, NH4+ and BH4−, which are interlinked by a dense network of dihydrogen bonds. Here we report the synthesis, structure and properties of solid solutions in the binary systems, NH4BH4-MBH4 (M = K, Rb, Cs), which are investigated by in situ synchrotron radiation powder X-ray diffraction and thermal and photographic analysis. Full solubility and formation of (NH4)xM1−xBH4, is observed upon cryo-mechanochemical treatment. The solid solutions stabilize NH4BH4 from T ∼68 to ∼96 °C, alter the decomposition pathway and suppress the fierce decomposition of NH4BH4. However, for increased amounts of NH4BH4 in the solid solutions, the decomposition gradually shows more resemblance to that of pristine ammonium borohydride, and the thermal stability of the solid solutions appears to decrease down the group of the alkali metal ions, i.e.