New ternary Mg–Mn–Ni alloys synthesised by high energy ball milling have been studied as hydrogen storage materials. A new Mg 3 MnNi 2 ternary intermetallic compound of the Ti 2 Ni type has been identified, its structure was studied by neutron diffraction. It has been shown that in the Mg–Mn–Ni system this intermetallic coexists with Mg, forming two-phase Mg–Mg 3 MnNi 2 alloys. The Mg 3 MnNi 2 compound reversibly absorbs about 1 wt.% of hydrogen already at room temperature and forms interstitial type Mg 3 MnNi 2 H ∼3 hydride. Substantially higher hydrogenation capacity has been observed in Mg–Mg 3 MnNi 2 alloys with high Mg content, e.g. 5.4 wt.% for the Mg 88 Mn 4 Ni 8 alloy at 250–350 °C. Hydrogen absorption–desorption kinetics of these alloys are drastically improved compared to undoped magnesium due to catalytic effect of the Mg 3 MnNi 2 phase.