A recent single-crystal study determined the crystal structure of MgSO4.3D2O, and discovered that a first-order phase transition occurs when the crystal is cooled below 245 K. The symmetry of the low-temperature phase could not be determined. The objective of this proposed work is (i) to make precise measurements of the unit-cell parameters of MgSO4.3D2O as a function of temperature, since the single-crystal data only permit comparatively imprecise refinement of axial ratios, and (ii) to characterise the change in symmetry that occurs on passing through the first-order phase transition at 245 K.Magnesium sulfate trihydrate is more stable than most other metastable hydrates of MgSO4, and may be the most likely phase to occur on the surface of Mars.