Two reactive hydride composite systems, Ca(BH4)2–NaNH2 and Mg(BH4)2–NaNH2, were systematically studied by in situ synchrotron radiation powder diffraction, in situ Fourier transform infrared spectroscopy, thermogravimetric analysis and differential scanning calorimetry coupled with mass spectrometry. Metathesis reactions between the amides and borohydrides take place in both systems between 100°C and 150°C yielding amorphous materials with the proposed composition M(BH4)(NH2). Simultaneously, a fraction of NaNH2 decomposes to Na3N and ammonia via a complex pathway. The main gas released under 300°C is ammonia for both systems, while significant amounts of hydrogen are released only above 350°C.