Duloxetine hydrochloride (1) is an important antidepressant that acts as a serotonin and noradrenaline reuptake inhibitor that has only recently been characterized by single-crystal X-ray diffraction. This study describes an investigation into polymorphism of duloxetine hydrochloride, discusses the challenges of characterizing new structures, and reports a new metastable solvate (1(acetone)) where acetone is trapped in a duloxetine hydrochloride host lattice. In view of the importance of formulation processing and bioavailability characteristics of the crystalline forms of 1, a comprehensive structural study of 1(acetone) was carried out using single-crystal and powder X-ray diffraction, infrared and Raman spectroscopies, and solid-state NMR spectroscopy. The rapid desolvation from 1(acetone) to the stable unsolvated form was investigated, and the structures of free and solvated forms are discussed in terms of the noncovalent intermolecular interactions.