Complex metal hydrides progressively display improved hydrogen storage capacity, but they are still far from fulfilling the requirements of the transport sector. Recently, indirect storage of hydrogen as ammonia in Mg(NH3)(6)Cl-2 has shown impressive capacity and reversibility. Here, we present an in situ nanoscale structural characterization of the thermal decomposition of Mg(NH3)(6)Cl-2 using small angle X-ray scattering (SAXS). We observe the growth of polydisperse spherical Mg(NH3)(2)Cl-2 crystallites forming a skeletal structure, the subsequent agglomeration of MgCl2 and formation of a nanoscale porosity consisting of 25-30 nm wide channels, which may account for the exceptional fast reloading of the material. (c) 2007 Elsevier B.V. All rights reserved.