The electronic properties of the Mg2NiH4 monoclinic phase are calculated using a density functional approach calculation. The crystalline parameters and interatomic distances calculated are close to the experimental values within a 3% error. We also evaluate the density of states (DOS) and character of the chemical bonding for the hydrogen's located in their equilibrium positions. While the Ni - Mg interaction is dominant in the pure alloy, in the hydride the hydrogen atoms present a bonding much more developed with Ni than with Mg. The principal bonding interaction is Ni sp - H s. Moreover, a small bonding between Ni d(eg) and H Is is observed. Up the Fermi level, the Ni - H interaction is slightly antibonding. The Mg - Ni bonding interactions are weaker in the hydride phase when compared with the pure Mg2Ni alloy. The present study is potentially useful because the alloys Mg - Ni are good materials for hydrogen storage. (c) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.