Mixtures of XMg-Co containing different amounts of Mg (X = 2, 3 and 7) were reactive milled under hydrogen atmosphere. 2Mg Co only formed the Mg2CoH5 complex hydride, while the mixtures 3Mg Co and 7Mg Co formed different contents of Mg2CoH5 and MgH2. Their structural features and hydrogen storage properties were analyzed by different techniques. In-situ synchrotron X-ray diffraction, combined with thermal analysis techniques, (differential scanning calorimetry, thermal gravimetric analysis and quadrupole mass spectrometer) was carried out to observe the behavior of the MgH2-Mg2CoH5 mixtures during the first H-desorption. It was found that the presence of the Mg2CoH5 complex hydride has a beneficial effect on the first H-desorption of the MgH2. Additionally, after first desorption, conventional hydrogenation under high pressure and high temperature of 3Mg Co and 7Mg Co samples led to the formation of the Mg6Co2H11 complex hydride. The presence of Mg6Co2H11 considerably impaired the desorption properties of the nanocomposites. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.