A manganese borohydride, Mn(BH4)(2), co-existing with a nanocrystalline LiCl salt, which is a reaction "dead-weight" byproduct, was successfully synthesized by the mechano-chemical activation synthesis (MCAS) during ball milling the (nLiBH(4) + MnCl2) mixtures having the molar ratios n = 2 and 3, using the total milling energy input, Q(TR), from 36.4 to 364 kJ/g. The crystallite (grain) size of the synthesized nanocrystalline Mn(BH4)(2) hydride attains 21 +/- 5.0 nm for the energy input Q(TR) = 36.4 kJ/g and then it is further reduced to 18 +/- 1.0 nm for Q(TR) = 145.6 kJ/g and finally to 14 +/- 0.5 nm for Q(TR) = 364 kJ/g. The crystallite (grain) size of LiCl is very close to 30 nm regardless of the milling energy input, Q(TR). During continuous heating in a Differential Scanning Calorimeter (DSC), Mn(BH4)(2) decomposes in endothermic reaction releasing H-2 and forming amorphous Mn and B in the process. The synthesized nanocrystalline Mn(BH4)(2) hydride, co-existing with a nanocrystalline LiCl salt, is capable of desorbing up to similar to 4.5 wt.% at 100 degrees C. The values of the apparent activation energy for dehydrogenation obtained in the present work are very low. The apparent activation energy for the n = 3 nanocomposite decreases monotonically from similar to 70 to similar to 59 kJ/mol with increasing milling energy input whereas the apparent activation energy for the n = 2 nanocomposite decreases from about 65 kJ/mol for Q(TR) = 36.4 kJ/g to about 53 kJ/mol for Q(TR) = 145.6 kJ/g and then again increases to similar to 59 kJ/mol for the Q(TR) = 364 kJ/g. These changes closely follow the variations in the average powder particle size obtained with the varying milling energy input. For the milling energy input Q(TR) = 36.4 and 145.6 kJ/g the average powder particle size decreases to 14.9 +/- 6.6 and 7.5 +/- 2.6 mu m, respectively, and subsequently increases reaching the average size of 16.1 +/- 6.3 mu m for the milling energy input Q(TR) = 364 kJ/g. On the other hand, the apparent activation energy for dehydrogenation doesn't depend on the average crystallite (grain) size. The amorphous Mn and B elements are also formed after isothermal dehydrogenation. The synthesized Mn(BH4)(2) hydride is very stable and doesn't excessively release H-2 during a long-term storage at room temperature for over 120 days under a slight overpressure of argon. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.