A brief description is presented of problems concerning the realisation of hydrogen reservoirs for vehicles powered by on board fuel cells. Hydrogen storage in the solid state, e.g. in hydrides, is recognised to be the best perspective and then the requisites for a metal hydride to be used with this purpose are discussed. The problems to be resolved in order to obtain a metal hydride interesting for applications concern both thermodynamics and kinetics of hydrogen absorption/desorption processes. The thermodynamics aspects for the formation of an hydride from a metal or an alloy and gaseous hydrogen are described by the pressure-composition isotherms (PCI). These PCI curves are characterised by a plateau, for every given temperature, in correspondence to the equilibrium pressure at which hydrogen can be reversibly stored. The equilibrium pressure strongly depends on the enthalpy variations, according to the van't Hoff graph. When two or more hydrides are formed at a given temperature by sequential increase of the pressure, two or more plateaux will appear in the corresponding PCI diagram. For an ideal hydride, working pressure and temperature should lie in the range 1 to 10 bar and 20 to 100 deg C, respectively, corresponding to enthalpy variations in the range of 15 to 24 kJ/mol_H. The process kinetics can be improved by particular treatments of the starting material such as ball milling (to greatly enhance the surface area and the density of structural defects) and by the addition of catalysts. Moreover, for on board applications, the weight ratio hydrogen/reservoir should be rather high, implying the use of hydrides of light metals, as Mg. The problem with Mg is the very slow kinetics, which significantly improves by ball milling, and the high working temperature (about 300 deg C). In this work preliminary results are reported on microstructural changes of magnesium hydride by ball milling treatment and on the effect of Nb addition on mechanics and temperature of hydrogen desorption. Samples of commercial MgH2 have been milled with a Spex8000M mill and reacted with hydrogen in a Sievert apparatus by performing a number of desorption/absorption cycles until a maximum storage capacity is reached (activation). Structural characterisation of as received, milled and activated samples was performed by X-ray diffraction. After activation a significant increase of grain size and decrease of desorption temperature are observed in MgH2 samples with Nb addition. The catalytic effect of Nb addition is also evidenced by the reduction of the number of activation cycles and by the lower temperature at which desorption starts, as measured by thermal desorption spectra. Finally it is shown that when Nb is added the desorption kinetics is determined by the hydrogen recombination at the surface.