l-Histidinium dihydrogenarsenate orthoarsenic acid (LHA) crystal ; l-HistH + .H 2 AsO 4 - .H 3 AsO 4 ; belongs to P2 1 space group of the monoclinic system, Z=2, a=9.264(2) 9, b=8.929(2) 9, c=8.874(2) 9, β=108.61(3) o . The crystal is isomorphous to l-HistH + .H 2 PO 4 - .H 3 PO 4 (LHP) crystal. The H 2 AsO 4 - anions and H 3 AsO 4 molecules are joined into layers parallel to the (100) crystallographic planes by the O-H...O type hydrogen bonds. The histidinium cations occupy the space between the layers. The histidinium cation posses the carboxylic COO - group. The positive charges appear in the ring (the proton is attached to the N(3)- nitrogen) and in the ammonium group. The differential scanning calorimetric (DSC) diagram does not show any phase transition till 100K. The infrared spectrum fully confirms the X-ray crystal structure. The LHA crystal is a good second harmonic generator (0.46 relative to KDP)