Form IV was found on decompression of Form III from 2.6 GPa to 1.7 GPa, like Form II it has two molecules in the asymmetric unit. Both molecules have gauche N-C-C-S dihedral angles, one g+ and the other g-. The hydrogen atoms bonded to sulfur, however, are not resolved in the X-ray diffraction experiment and therefore the work presented here on Form IV has assumed initial hydrogen positions based on those hydrogen bond acceptors (S or O) which are nearest to the donor sulfur atom. This resulted in one of the symmetry unique molecules being involved in an S-H...S hydrogen bond and the other with an S-H...O hydrogen bond in a similar way to Form II. Details of the unit-cell dimensions and the pressures and temperatures at which they were determined is provided in the Supplementary Information (SI), Table -1. This table also shows the nature of the hydrogen bonding found in each polymorph and illustrates that a significant factor in the differences between the polymorphs and the effect of temperature is the C-C-S-H dihedral angle, which in turn dictates whether the hydrogen bonding pattern is SH...O or SH...S. As can be seen from the Table, Form I has been studied 6 over a wide range of temperatures from 30 K to 300 K. Some disorder in the hydrogen bonding pattern is seen at all temperature above 105 K; only the crystal structure of Form I at or below 30 K shows a completely ordered SH...S bonding pattern. For the NH...O hydrogen-bond motifs, which are also found in all the polymorphs, the article on hydrogen-bond motifs in hydrophobic amino acids provides a summary of the common types found in these systems. Previous Density Functional Theory (DFT) molecular dynamics calculations have been performed to understand the terahertz spectrum of l-cysteine, which was determined at 77 K and 298 K. Static solid state energy calculations 5 indicated that the Form I polymorph with S-H...S intermolecular interactions is only 4.1 kJ mol -1 lower in energy than a similar crystal structure but with S-H...O intermolecular bonding, confirming the likelihood of disorder as observed experimentally.