0.75Mg-0.25TM-H (TM = Ti, Nb or V) samples were mechano-chemically synthesized by reactive ball milling. The detailed reaction mechanism during hydrogen release and uptake was investigated by in-situ synchrotron radiation powder X-ray diffraction (SR-PXD) experiments. The thermodynamic and kinetic properties have been studied by Sievert's method. On the base of calculated values of the Gibbs free energy for reaction of hydrogen absorption (Delta G < 0) it reveals that hydrogenation reaction could thermodynamically proceed at room temperature, which is experimentally confirmed for all of the studied composites. It is clearly shown that beta-MgH2 forms at RT under conditions of experiment. Comparative analysis of the Mg-Ti, Mg-V and Mg-Nb systems makes it possible to establish that the most effective additive facilitating hydrogen uptake, particularly at RT, is vanadium. It provides the degree of conversion into hydride phase alpha = 0.86 for the first minute of hydrogenation. In contrast, additives of Nb and Ti provide only alpha = 0.62 and 0.36, respectively, following the 30 min of exposure. However, this study reveals that for the dehydrogenation process, titanium is the best among the examined additives, which is evidenced by lowest value of activation energy E-a = 53.6 kJ/mol of the hydrogen desorption. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.