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DuP 747, a selective kappa agonist analgesic, was found to have at least two polymorphic forms, and this was confirmed by X-ray powder diffraction. DSC and thermomicroscopic studies indicated the polymorphic pair to be monotropic. Solubility studies suggested the relative stability of the two forms to be similar. The infrared (IR) and Raman spectra of the two crystal forms were significantly different, and their complementary nature was shown from the differences in peak intensities. Solid-state 13C-NMR data of the polymorphs showed only minor differences between the two forms. When viewed on the molecular level through the use of vibrational and NMR spectroscopies, the conformation of the molecule in the two polymorphs appears to be roughly equivalent. The magnitude of the spectral differences of the two polymorphs is, however, consistent with those that can be expected for two crystal forms that have resulted from different modes of packing, as caused by the solvent environment.