The amino acid l-cysteine (Figure ) is known to crystallise in two polymorphs at ambient pressure. Form I is orthorhombic (P 2 1 2 1 2 1 ) 1 with one molecule in the asymmetric unit and Form II is monoclinic (P 2 1 ) 2 with two molecules in the asymmetric unit. The crystal structures of both forms have been studied at ambient temperatures; Form I by X-ray 1 and neutron diffraction and Form II by X-ray diffraction. An X-ray diffraction study of Form I at 30 K has also been reported. In all these studies l-cysteine is found as a zwitterion (see Figure ).