We describe an unusual application of synchrotron energy-dispersive diffraction with hard X-rays to obtain structural information on metastable sodium sulfate heptahydrate. This hydrate was often mentioned in nineteenth and early twentieth century scientific literature but rarely in modern publications, and it had not been characterised structurally. Using a unique three-detector fixed-angle X-ray geometry, a good quality powder diffraction pattern was obtained directly from a stirred suspension of hydrate crystals in saturated aqueous sodium sulfate solution at about 14 °C. The suspension of crystals was contained in the 22 mm dia sealed cylindrical bottle in which crystallization occurred. Indexing showed that the heptahydrate has a tetragonal unit cell with a = 7.1668 Å and c = 22.2120 Å with a few weak unindexed reflections arising from the 2a supercell. New gravimetric data and the cell dimensions confirm the heptahydrate composition originally proposed by Loewel.