The crystal structures of L-arginine(.)2HCl(.)H(2)O (I) and L-arginine(.)2HBr(.)H(2)O (II) are determined. Both structures contain double charged arginine cation [(+)(H2N)(2)CNH(CH2)(3)CH(NH3+)COOH]. In contrast to the corresponding isomorphous monohalide, monohydrates the crystals (I) and (II) are not isomorphous and crystallize in orthorhombic (space group P2(1)2(1)2(1)) and triclinic (space group P1) systems, respectively. Crystals (I) and (II) differ from each other also in respect to interaction of cations with each other, with anions and with water molecule. Crystal (II), having polar symmetry, displays strong pyroelectric effect. (C) 2003 Elsevier B.V. All rights reserved.