THz-TDS was used to study the crystallisation process of MgSO 4 •7H 2 O. The emergence and disappearance of the spectral feature at 1.6 THz indicated the growth or dissolving of crystals in the field of view of the spectrometer (validated by image analysis), while the change of the baseline reflected the behaviour of solvent. This is useful for investigating solvation dynamics and the behaviour of molecular species at phase boundaries.