The pressure dependence of the Raman scattering of Form I has also been examined up to 20.2 GPa. Two structural transitions were observed; one between 1.8 and 3.6 GPa and the other between 6.9 and 7.8 GPa. The first is probably associated with interconversion between the two types of S-H hydrogen bonds and the second with the break-down of these hydrogen bonds. A further 3 transitions were suggested by the data. The temperature induced phase transition of Form I has been studied using DSC and powder X-ray diffraction in the temperature range from 268 K to its melting point 493 K. Two phase transitions were seen at 464 K and 480 K.