Ternary Mg-based mixtures (50 <= Mg weight% <= 80) containing increasing amount of Ni (up to 30 wt%) and Cu (up to 20 wt/o) have been prepared by ball milling in hydrogen reactive atmosphere (P-H2 = 5 bar) to determine the effects of both the processing conditions and the concurrent presence of the two transition metals on the sorption mechanisms and the storage properties of the Mg/MgH2 system. Combined SEM, TG and XRPD analyses showed that the processing time t(BM) (=3 h, 8 h and 16 h) strongly affects the properties of the "as milled powders" (average particle sizes and microstructure, nature of the phases, amount of hydrogen absorbed upon milling, and desorption characteristics). However, after a combined high temperature/high pressure activation cycle, all the charged samples were composed of MgH2, Mg2NiH4 and MgCu2 while the discharged samples contained "free Mg" and the intermetallic compounds Mg2Ni and Mg2Cu ("bonded Mg"): in all cases the definitive storage performances are related only on the relative amounts of these three phases that, in turn, depend on the starting composition of the mixtures. Both Ni- and Cu-containing phases hydrogenated/dehydrogenated according to their own sorption reaction schemes; however, Mg2NiH4 and MgCu2 destabilized each other, with the beneficial effect of rising the desorption plateau pressures of the "bonded Mg" with respect to binary Mg-Ni and Mg-Cu mixtures. On the contrary, Ni- and Cu-containing phases did not exert any appreciable destabilizing effect towards "free MgH2". (C) 2008 Published by Elsevier Ltd on behalf of International Association for Hydrogen Energy.