Hydride nanocomposites in the (LiNH2 + nMgH(2)) system have been synthesized by ball milling with varying input of milling energy injected into powder particles, Q(TR) (kJ/g). The grain (crystallite) size of LiNH2 and MgH2 decreases rapidly with increasing Q(TR) up to approximately 150-200 kJ/g and subsequently more or less saturates at the value of 10-20 nm. For the injected energy Q(TR) approximate to 250-350 kJ/g the specific surface area (SSA) increases from the initial 2.4 m(2)/g for powder mixtures before milling to 30-37 m(2)/g for nanocomposites after milling. After injecting Q(TR) approximate to 550 kJ/g there is a further increase of SSA to 52 m(2)/g which is over 20-fold increase of SSA from its initial value. That clearly indicates that a profound reduction of particle size has occurred. The hydride phases formed during ball milling with relatively low Q(TR) are identified as a-Mg(NH2)(2) (amorphous magnesium imide) and LiH. The ball milled (LiNH2 + nMgH(2)) nanocomposite system with n = 0.5-0.9 can effectively desorb about 4-5 wt.% H-2 with a reasonable rate at the temperature range close to similar to 200 degrees C. Within a low temperature range up to 250 degrees C, regardless of the molar ratio n and the injected energy Q(TR) the thermal desorption of the (LiNH2 + nMgH(2)) nanocomposites occurs without any release of ammonia, NH3. For all molar ratios, n, the hydride nanocomposites are fully reversible at 175 degrees C under a relatively mild pressure of 50 bar H-2. The quantity of H-2 desorbed decreases with increasing molar ratio n, due to increasing fraction of inactive, retained MgH2. However, at 125 degrees C the dehydrogenation rate is very sluggish and the quantity of released H-2 is minimal. At the temperature range lower than similar to 250 degrees C dehydrogenation of ball milled nanocomposites occurs through formation of the Li2Mg(NH)(2) hydride phase. The value of the measured dehydrogenation enthalpy change of 46.7 kJ/molH(2) is relatively low and apparently, it is not responsible for sluggish dehydrogenation at 125 degrees C. The measurements of thermal conductivity for non-milled powders and ball milled nanocomposites show a dramatic reduction of thermal conductivity after ball milling. It seems that this could be a principal factor responsible for such a low dehydrogenation rate at low temperatures. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.