Thermochemical reactions between alkali metal amides and magnesium hydride taken in 2:3 molar ratios have been investigated using pressure-composition-temperature, X-ray powder diffraction and residual gas analysis measurements. The thermally induced reactions in both title systems are stoichiometric and proceed as a following solid state transformation: 2MNH(2)+ 3MgH(2) -> Mg(3)N(2) + 2MH + 4H(2)up arrow. A total of 6.45 wt.% of hydrogen is released by the 2LiNH(2)-3MgH(2) system beginning at 186 degrees C, and a total of 5.1 wt.% H(2) is released by the 2NaNH(2)-3MgH(2) system starting at 130 degrees C. Combined structure/property investigations revealed that the transformation in the lithium containing system proceeds in two steps. In the first step, lithium amide reacts with MgH(2) to form Li(2)Mg(NH)(2) and hydrogen. In the second step, reaction between Li(2)Mg(NH) and MgH(2) leads to the formation of the Mg(3)N(2) nitride, lithium hydride and additional gaseous hydrogen. The transformation in the sodium containing system appears to proceed through a series of competing solid state processes with formation of Mg(NH(2))(2) and NaMgH(3) intermediates. Partial rehydrogenation in 190 bar hydrogen pressure leading to formation of the MgNH imide was observed in the dehydrogenated 2NaNH(2)-3MgH(2) system at 395 degrees C. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.