The crystal structures of the 1:1 pyrene-pyromellitic dianhydride molecular compound at 110 and at 300 °K have been determined by three-dimensional X-ray diffraction methods (visual intensity measurement, least-squares refinement with isotropic Debye-Waller factors). The component molecules are arranged, plane to plane, in alternating array in mixed stacks. At 110 °K the pyrene molecules in the stacks alternate in two different orientations (about 12° apart) with respect to the interleaving pyromellitic dianhydride molecules, which themselves have small displacements of alternating sign off the stack axis. At about 200 °K this ordered structure changes to a disordered structure which has been determined at 300 °K. The component molecules appear in mean positions, abnormally large Debye—Waller factors indicating disorder. The transformation itself has not yet been studied in detail. Review of the literature suggest that the existence of positional disorder provides an explanation for the widespread reports of high degrees of thermal vibration in crystals of 77-molecular compounds. Some structure determinations appear to have been of disordered structures without this having been fully appreciated. Analysis of Debye-Waller factors, electron-density and difference syntheses suggests that different types of disorder are found in pyrene-pyromellitic dianhydride and pyrene- 1,3,7,9-tetramethyluric acid (Damiani et al. 1965) on the one hand and in naphthalene-tetracyanoethylene (Williams & Wallwork 1967) and naphthalene-1,2,4,5-tetracyanobenzene (Kumakura, Iwasaki & Saito 1967) on the other. The dimensions found for the planar pyrene molecule at 110 °K agree well with those in pyrene itself but are more accurate; the agreement between the present experimental bond lengths and those calculated by the simple valence-bond method is exceptionally good (r.m.s. discrepancy = 0*004 A). The dimensions of the pyromellitic dianhydride molecule agree well with previous results but are more accurate; the molecule is slightly twisted into a non-centrosymmetric shape.