Magnesium complex hydrides are attractive for hydrogen storage applications, mainly due to their high volumetric capacities and to their relatively low cost. In this work, nano-crystalline Mg2CoH5 was synthesized with very high yields (97%) by reactive milling cobalt and magnesium under relatively mild processing conditions (30 bar of H-2 pressure and 12 h of milling). The behavior of the milled Mg2CoH5 during heating was studied by a combination of several techniques including DSC, QMS, TGA and in-situ synchrotron XRD. It is shown for the first time that two different mechanisms of hydrogen desorption take place. At low temperatures (up to 325 degrees C), some hydrogen is released by a diffusional mechanism with no change in the crystalline structure of the high temperature gamma-Mg2CoH5 phase. At higher temperatures, above 325 degrees C, the gamma-Mg2CoH5 phase becomes unstable and the complex hydride decomposes into Mg, Co and H-2. This is the first work to report the diffusional hydrogen desorption mechanism for the Mg2CoH5 or any other complex hydride. Furthermore, a complete description of the allotropic beta-Mg2CoH5 to gamma-Mg2CoH5 phase transition is provided. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.