Abstract Two new magnesium–niobium hydrides, Mg 6.5NbH ∼14 with a face centred cubic unit cell a = 9.548(1) Å (space group F m 3 ¯ m (No. 225), Z = 4, V = 870.49 Å 3) and MgNb 2H ∼4 with a C-centred monoclinic unit cell a = 5.685 (4) Å, b = 3.2914(6) Å, c = 7.924(2) Å and β = 103.82(3)° (space group C2/ m (No. 12), Z = 2, V = 143.98 Å 3), were synthesized by reacting MgH 2 and Nb in a high-pressure anvil cell at 8 GPa and 600 °C. The structures of the new hydrides were refined from synchrotron radiation data. Mg 6.5NbH ∼14 has a structure related to Mg 7TiH ∼16 ( a = 9.532(2) Å space group F m 3 ¯ m (No. 225)) (D. Kyoi, T. Sato, E. Rönnebro, N. Kitamura, A. Ueda, M. Ito, S. Katsuyama, S. Hara, D. Noréus, T. Sakai, J. Alloys Compd. 372 (2004) 213). Where the metal atom structure is of the Ca 7Ge type. Mg 6.5NbH ∼14, however, has some vacancies on one of the magnesium sites similar to the recently discovered Mg 6VH ∼14 (D. Kyoi, T. Sato, E. Rönnebro, Y. Tsuji, N. Kitamura, A. Ueda, M. Ito, S. Katsuyama, S. Hara, D. Noréus, T. Sakai, J. Alloys Compd. 375 (2004) 253). The metal atoms in MgNb 2H ∼4 have the AuTe 2 type structure.