p-diiodobenzene exists in two crystalline phases, both of which are orthorhombic, with space groups Pbca (α phase) and Pccn (β phase), respectively. The α phase is the stable form at ambient temperature, and a phase transition to the β phase occurs at ca. 326 K. This phase transition is associated with interesting hysteresis and metastability phenomena. From previous diffraction studies, the crystal structure of the α phase has been determined. In this work, we present a first structure determination of the β phase, from neutron powder diffraction data. It is shown that the structure of the β phase differs from the structure of the α phase by a change (ca. 46°) in the orientation of half of the molecules. This change in molecular orientation does not significantly alter the overall molecular packing arrangement (a fact consistent with the small change in lattice parameters and the small enthalpy change associated with the phase transition). The differences and similarities between crystal structures of the α and β phases are discussed in detail. The temperature-dependent structural properties of p-diiodobenzene have also been investigated by high-resolution solid state 13C NMR spectroscopy. Detailed results from these investigations are reported and discussed in the light of the results from the neutron diffraction studies. Furthermore, in view of the fact that only rather small structural differences exist between the α and β phases of p-diiodobenzene, our results have wider implications from the viewpoint of assessing the scope of solid state NMR spectroscopy as a technique for probing minor changes in structural properties of crystalline organic solids. © 1994 Academic Press, Inc.