Laboratory X-ray powder diffraction data were used to investigate the dehydration process of magnesium bromide hexahydrate in the temperature range 300 K ≤ T ≤ 420 K. By heating of the as synthesized hexahydrate (MgBr2·6H2O, observed in the temperature range 300 K ≤ T ≤ 349 K), three lower hydrates can be obtained in overlapped temperature regions: MgBr2·4H2O (332 K ≤ T ≤ 367 K), MgBr2·2H2O (361 K ≤ T ≤ 380 K) and MgBr2·H2O (375 K ≤ T ≤ 390 K). Although the crystal structure of the hexahydrate was published almost eighty years ago, there are no data on the structures of the lower hydrates. The crystal structures are reported and are found to be isotypical with the structures of the respective chlorides. The structure of MgBr2·6H2O is characterized by discrete Mg(H2O)6 octahedra and is the only hydrate of this group that contains unbonded Br– anions. MgBr2·4H2O is composed of discrete MgBr2(H2O)4 octahedra, and the structure was found to be disordered. The crystal structure of MgBr2·2H2O is formed by single chains of edge-sharing MgBr4(H2O)2 octahedra, while in the case of MgBr2·H2O double chains of edge-shared MgBr5H2O are formed. By increasing the temperature, as expected, positive thermal expansion was evidenced. Thermal expansion coefficients, based on the changes of the unit cell parameters, were derived for the following hydrates: MgBr2·6H2O, MgBr2·4H2O, and MgBr2·2H2O.