The terahertz spectra of MgSO 4 anhydrous, monohydrate, heptahydrate, and the crystal grown in the lab from solution were acquired at different temperatures (Figure ). Comparing the spectra of the three different hydrates, neither the anhydrous nor monohydrate forms of MgSO 4 showed pronounced peaks in the region of interest (0.3 THz to 3.0 THz), and the only change that was observed upon cooling was a drop in the baseline. For the commercial heptahydrate sample, three pronounced bands were observed at 80 K: at 1.2 THz, 1.7 THz (double features), and 2.8 THz. These vibrations probably resulted from the interactions between MgSO 4 and water because they were not present in the anhydrous and monohydrate samples. As expected, the spectra exhibited peak broadening and shifting as well as an increase of the baseline upon heating to room temperature. This is due to the significant population of excited vibrational states at room temperature, which is characteristic of the far-infrared where the energy gap between ground state and excited states is lower than in the infrared. In addition, the increased thermal vibration and emission contribute to this effect. At 294 K, which was close to the temperature of the crystallisation experiments, the high intensity peak at 2.8 THz diminished into the baseline and the two features at lower frequencies became weaker and broader while the double peak at 1.7 THz merged and shifted to a single feature at 1.6 THz.