The hydrogen desorption from MgH2-Mg2NiH4 phase mixtures prepared under different ball milling conditions has been investigated and correlated with the samples microstructure. SEM, XRD and XPS were performed to explore the microstructure in order to ascertain the reciprocal influence of the two phases on the de-hydrogenation reaction, and in particular on the hydrogen desorption temperature. Ball milling imparts different microstructures to the phase mixtures according to the adopted processing parameters. The resulting microstructure influences the hydrogen desorption which appears to occur by a limited number of well-defined channels at characteristic temperatures. A microstructural interpretation of this behavior is proposed on the basis of the characterization results. From the applicative point of view it is possible to notice that this mixture, when property processed, shows a onset temperature of the desorption of the whole hydrogen content about 150K lower than MgH2, even on samples exposed to the atmosphere. (c) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.