A novel low-temperature hydride, Mg 3 MnNi 2 D 3−x with filled Ti 2 Ni-type structure has been synthesised and its crystal structure has been studied in detail by powder neutron diffraction. Refinement of the structures of β-Mg 3 MnNi 2 D 1.83 and α-Mg 3 MnNi 2 D 0.23 deuteride phases showed that D atoms partially fill 16d Mg 6 octahedral and 32e Mg 3 Ni tetrahedral interstices in the cubic metal matrix (sp.gr. Fd3¯m). The β-hydride was found to be rather unstable, during storage in air it transforms into α-hydride.