At the beginning of the experiment, both the terahertz spectra and visual analysis confirmed the absence of MgSO 4 crystals located in the centre of the cell. Once nucleation occurred, typically not in the centre of the cuvette but near one end of the cell, a local increase in water concentration was observed in the terahertz spectra due to the increase in water concentration immediately adjacent to the growing crystals as magnesium and sul- The colour denotes the temperature at which the system was kept during crystallisation. MgSO 4 in solution is surrounded by a hydration shell whose absorption is markedly different from that of bulk water. This was demonstrated by calculating a theoretical absorption coefficient based on the known absorption coefficient of pure water (α water ) and that of anhydrous MgSO 4 and that of MgSO 4 •7H 2 O (α crystal ), neglecting the effect of a larger hydration shell. A series of MgSO 4 aqueous solution with a range of concentrations were measured, and a difference between the measured (α solution ) and the calculated (α ideal mixture ) absorption was consistently observed.