In the newly developed set up, a closed microfluidics platform made of z-cut quartz was utilised which enables investigation of both the solvent and solute at frequencies from 0.35 THz to 2 THz whilst allowing for optical observations in the visible range of the spectrum during crystallisation and for cleaning of the cell between experiments. Magnesium sulphate hydrates were chosen as a model system, as different hydrates are known to form under specific conditions. Between 2 °C to 47 °C, MgSO 4 •7H 2 O is the predominant form that can be crystallised at reasonable experimental time scales . Therefore, MgSO 4 •7H 2 O is used as an example to demonstrate the setup designed based on transmission THz-TDS that can observe the crystallisation and characterise the structure before, during, and after crystal formation in situ.