p-Phenylenediamine can be obtained as the dihydrate, C(6)H(8)N(2) x 2 H(2)O, (I), and in its anhydrous form, C(6)H(8)N(2), (II). The asymmetric unit of (I) contains one half of the p-phenylenediamine molecule lying about an inversion centre and two halves of water molecules, one lying on a mirror plane and the other lying across a mirror plane. In (II), the asymmetric unit consists of one molecule in a general position and two half molecules located around inversion centres. In both structures, the p-phenylenediamine molecules are arranged in layers stabilized by N-H...pi interactions. The diamine layers in (I) are isostructural with half of the layers in (II). On dehydration, crystals of (I) transform to (II). Comparison of their crystal structures suggests the most plausible mechanism of the transformation process which requires, in addition to translational motion of the diamine molecules, in-plane rotation of every fourth p-phenylenediamine molecule by ca 60 degrees. A search of the Cambridge Structural Database shows that the formation of hydrates by aromatic amines should be considered exceptional.