Pure Mg(BH4)2 has been characterized by single-crystal and powder synchrotron X-ray diffraction and by vibrational spectroscopies. The earlier reported P61 structure of the α-phase is revised in the space group P6122. Location of the H-atoms from powder data in the published P61 models posed the main problem for the identification of the correct symmetry: wrongly determined orientations of some BH4 groups hampered a successful detection of the true P6122 symmetry. Four nearly ideally tetrahedral BH4 groups form a dodecahedral MgH8 coordination around each Mg atom, which can be described as a slightly distorted snub disphenoid, where two nearly planar BH2−Mg−H2B fragments are situated at ~ 90° dihedral angle. The H···H distances between two BH4 groups within the BH2−Mg−H2B fragments are among the shortest (2.18−2.28 Å) in the structurally characterized metal borohydrides. α-Mg(BH4)2 contains an unoccupied void, accounting for 6.4% of space in the structure. It is large enough (37 Å3) to accommodate a small molecule, such as H2O. The high-temperature β-phase is less dense by ~ 3% but contains no unoccupied voids. The α-phase transforms into the β-phase above 490 K; the latter is quenched (metastable) on cooling. The anomalous cell expansion of the β-phase down to 100 K may be related to the evolution of the free energy profile far from the phase transition temperature.