The effects of Ni and Co addition on the hydrogen storage properties of Mg(3)Mm alloy was studied by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX) and pressure-composition isotherm (PCI) measurement. The hydrogen absorption kinetics and the thermodynamic parameters (apparent Delta H, Delta S) for Mg(3)Mm dehydrogenation reactions in Mg(3)Mm, Mg(3)MmNi(0.1) and Mg(3)MmNi(0.1)Co(0.1) alloys have been also investigated. The maximum hydrogen storage content of Mg(3)Mm, Mg(3)MmNi(0.1) and Mg(3)MmNi(0.1)Co(0.1) alloys was improved due to that the addition of Ni and/or Co further spurred the MmH(3) phase transforming to MmH(2) phase. On the other side, the kinetics curves show the addition of Co could enhance hydrogen absorption rate while the addition of Ni change the hydrogenation reaction mechanism. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.