3,4-Diaminopyridine is an active pharmaceutical ingredient for the treatment of Lambert−Eaton myasthenic syndrome (LEMS). It is 3,4-diaminopyridine dihydrogen phosphate that has become the active ingredient of choice. As part of a larger study for the development of this drug, solid-state studies have been carried out. At room temperature, the crystals are monoclinic (C2/c). Dihydrogen phosphate anions H2PO4 − form infinite chains parallel to the b axis, and these chains can be considered as macroanions (H2PO4)∞. The organic cations form hydrogen bonds with the macroanions bridging them together. A negative thermal expansion is observed along the [103] direction in the crystal, which coincides with the direction of chains of interchanging anions and cations. The crystal packing shows similarities with the hydrogen tartrate salt as illustrated by a Hirshfeld surface analysis. This phosphate and this tartrate salt are also the two diaminopyridine salts that have been selected for therapeutic use because of their appropriate physical properties.