Manganese borohydride (Mn(BH4)(2)) was successfully synthesized by a mechano-chemical activation synthesis (MCAS) from lithium borohydride (LiBH4) and manganese chloride (MnCl2) by applying high energy ball milling for 30 mm. For the first time a wide range of molar ratios n = 1, 2, 3, 5, 9 and 23 in the (nLiBH(4) + MnCl2) mixture was investigated. During ball milling for 30 mm the mixtures release only a very small quantity of H-2 that increases with the molar ratio n but does not exceed similar to 0.2 wt.% for n = 23. However, longer milling duration leads to more H-2 released. For the equimolar ratio n = 1 the principal phases synthesized are Li2MnCl4, an inverse cubic spinel phase, and the Mn(BH4)(2) borohydride. For n = 2 a LiCl salt is formed which coexists with Mn(BH4)(2). With the n increasing from 3 to 23 LiBH4 is not completely reacted and its increasing amount is retained in the microstructure coexisting with LiCl and Mn(BH4)(2). Gas mass spectrometry during Temperature Programmed Desorption (TPD) up to 450 degrees C shows the release of hydrogen as a principal gas with a maximum intensity around 130-150 degrees C accompanied by a miniscule quantity of borane B2H6. The intensity of the B2H6 peak is 200-600 times smaller than the intensity of the corresponding H-2 peak. In situ heating experiments using a continuous monitoring during heating show no evidence of melting of Mn(BH4)(2) up to about 270-280 degrees C. At 100 degrees C under 1 bar H-2 pressure the ball milled n = 2 and 3 mixtures are capable of desorbing quite rapidly similar to 4 wt.% H-2 which is a very large amount of H-2 considering that the mixture also contains 2 mol of LiCl salt. The H-2 quantities experimentally desorbed at 100 and 200 degrees C do not exceed the maximum theoretical quantities of H-2 expected to be desorbed from Mn(BH4)(2) for various molar ratios n. It clearly confirms that the contribution from B2H6 evolved is negligibly small (if any) when desorption occurs isothermally in the practical temperature range 100-200 degrees C. It is found that the ball milled mixture with the molar ratio n = 3 exhibits the highest rate constant k and the lowest apparent activation energy for dehydrogenation, E-A similar to 102 kJ/mol. Decreasing or increasing the molar ratio n below or above 3 increases the apparent activation energy. Ball milled mixtures with the molar ratio n = 2 and 3 discharge slowly H-2 during storage at room temperature and 40 degrees C. The addition of 5 wt.% nano-Ni with a specific surface area of 60.5 m(2)/g substantially enhances the rate of discharge at 40 degrees C. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.