Triethylenetetramine is a polyamine type chelating ligand for copper(II), which is currently used, mainly in its dihydrochloride form, as an alternative to d-penicillamine against Wilsons disease. Because knowledge about the solid-state behavior of pharmaceuticals is a prerequisite for the development of an effective dosage form, the crystal structures of two triethylenetetramine dihydrochloride polymorphs have been determined, and the infrared spectra and thermal expansion have been studied. No suitable crystals could be obtained of the two anhydrous forms, for which the structures have been solved from X-ray powder diffraction. Form I is monoclinic P2(1)/n with the cell parameters a = 11.0475(4) angstrom, b = 4.87475(16) angstrom, c = 11.1178(4) angstrom, and beta = 106.9826(13)degrees leading to V = 572.63(3) angstrom(3) with Z = 2. Form II is also monoclinic P2(1)/c with cell parameters a = 10.6734(6) angstrom, b = 12.3613(6) angstrom, c = 9.8479(7) angstrom, and beta = 115.985(3)degrees leading to a unit-cell volume of V = 1167.95(12) angstrom(3) with Z = 4. A recurring structural element is a ionic interaction between two chloride ions and two ammonium groups each organized in a diamond pattern present in the dihydrate (the salt is hygroscopic) and anhydrous form I. The molecular conformations are different in both anhydrous forms and in the dihydrate. Comparison of the thermal expansions demonstrates that form I has the smallest thermal expansion and thus possesses the strongest intermolecular interactions.