A structural analysis of CaSO4 · 0.5H2O, a dehydration product of gypsum, has been carried out through the Rietveld method on X-ray powder diffraction data. A dehydrated powder of synthetic gypsum has been charged inside a non-hermetically sealed capillary in order to allow a slow rehydration. The starting material has been identified as γ-anhydrite, space group P6222, cell parameters a = 6.9691(2) Å, c = 6.3033(2) Å. The final product of the rehydration of γ-anhydrite is CaSO4 · 0.5H2O, space group I2 (unique axis b), cell parameters a = 12.0350(5) Å, b = 6.9294(3) Å, c = 12.6705(4) Å, β = 90.266(3)°. The structure of the hemihydrate is strongly pseudo-trigonal, space group P3121. The symmetry lowering arises from water molecules ordering inside the channels.