The crystal structure of the 2.5 hydrate MgSO4 phase was determined by simulated annealing from laboratory X-ray powder diffraction data measured from 2–140 °2𝛉 using CuKα radiation. The 2.5 hydrate is monoclinic, space group C2/c, with unit-cell parameters a = 18.8636(4) Å, b = 12.3391(2) Å, c = 8.9957(2) Å, β = 94.568(2)°, V = 2087.1(6) Å3, and Z = 16. The model was refined using fundamental-parameters Rietveld refinement, converging to Rwp = 8.89%, Rp = 6.61%, Rexp = 3.33%, RBragg = 3.95%, and χ2 = 2.67. The refined structure is consistent with a formula of 2.5 H2O. Bond-valence calculations for the refined model show that the structure is chemically sensible. In the refined structure, [Mg(O,H2O)6] octahedra and [SO4] tetrahedra build up 2-D double-sheet slabs by sharing vertex O atoms, which are held together by inter-slab H-bonds involving (SO4)2− groups and H2O molecules coordinated with Mg2+ cations to form the layer structure of the 2.5 hydrate phase.