In the present work we investigate the hydrogen sorption properties of composites in the MgH2-Ni, MgH2-Ni-LiH and MgH2-Ni-LiBH4 systems and analyze why Ni addition improve hydrogen sorption rates while LiBH4 enhance the hydrogen storage capacity. Although all composites with Ni addition showed significantly improved hydrogen storage kinetics compared with the pure MgH2, the fastest hydrogen sorption kinetics is obtained for Ni-doped MgH2. The formation of Mg2Ni/Mg2NiH4 in Ni-doped MgH2 composite and its microstructure allows to uptake 5.0 wt% of hydrogen in 25 s and to release it in 8 mm at 275 degrees C. In the MgH2-Ni-LiBH4 composite, decomposition of LiBH4 occurs during the first dehydriding leading to the formation of diborane, which has a Ni catalyst poison effect via the formation of a passivating boron layer. A combination of FTIR, XRD and volumetric measurements demonstrate that the formation of MgNi3B2 in the MgH2-Ni-LiBH4 composite happens in the subsequent hydriding cycle from the reaction between Mg2Ni/Mg2NiH4 and B. Activation energy analysis demonstrates that the presence of Ni particles has a catalytic effect in MgH2-Ni and MgH2-Ni-LiH systems, but it is practically nullified by the addition of LiBH4. The beneficial role of LiBH4 on the hydrogen storage capacity of the MgH2-Ni-LiBH4 composite is discussed. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.