Previous spectroscopic work reported on l-cysteine includes angle-dependent terahertz timedomain spectroscopy of single crystals and molecular quantum calculations of the vibrational modes of the cysteine molecule in its zwitterion form. This work showed a peak in the powder spectrum at 46.06 cm -1 , which was not observed in the single crystal spectrum. A complete assignment of the vibrational spectra of both Form I and Form II has been reported by using infrared, Raman, inelastic neutron scattering and solid state quantum mechanical calculations. Form II (monoclinic) is shown to have a a short SH...HN non-bonded interaction just less than twice the van der Waals' radius of hydrogen. Mink et al. have reported infrared spectra of Form I at room temperature in its various deuterated forms. The S-H and S-D stretching vibrations were found to be local modes, which were not sensitive to deuteration of the methylene or amino groups. The force constants of the S-H bond indicated that those involved in the SH...S hydrogen bonding network were stronger than those involved in the SH...O network. Terahertz measurements and static DFT calculations have been reported by Ren et al. who found 6 peaks in the low temperature spectrum Form I below 120 cm -1 and 3 peaks in the room temperature spectrum.