The fluorescence of six secondary arenedicarboxamides has been investigated in single crystals, powders, melts, and ethanol solution. Single crystals of four rodlike arenedicarboxamides which pack in one-dimensional tapes display dual emission attributed to excited-state monomers and dimers. These tapes have a constant 5-Angstrom separation between the long axes of neighboring arenes but differ in the arene-arene dihedral angle. The frequency shift between monomer and dimer fluorescence and the intensity and vibronic structure of the dimer emission are dependent upon the ground-state geometry. Single crystals of two naphthalenedicarboxamides which pack in two-dimensional sheets and have no close contacts between neighboring arenes within the same sheet display only monomer emission. Melted and resolidified samples display broad structureless emission attributed to fluorescence from a mixture of excited monomers and dimers with different ground-state geometries.