Both kinetics and thermodynamics properties of MgH2 are significantly improved by the addition of Mg(AlH4)(2). The as-prepared MgH2-Mg(AlH4)(2) composite displays superior hydrogen desorption performances, which starts to desorb hydrogen at 90 C, and a total amount of 7.76 wt% hydrogen is released during its decomposition. The enthalpy of MgH2-relevant desorption is 32.3 kJ mol(-1) H-2 in the MgH2-Mg(AlH4)(2) composite, obviously decreased than that of pure MgH2. The dehydriding mechanism of MgH2-Mg(AlH4)(2) composite is systematically investigated by using x-ray diffraction and differential scanning calorimetry. Firstly, Mg(AlH4)(2) decomposes and produces active Al*. Subsequently, the insitu formed Al* reacts with MgH2 and forms Mg-Al alloys. For its reversibility, the products are fully re-hydrogenated into MgH2 and Al*, under 3 MPa H-2 pressure at 300 C for 5 h. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.