The physical properties of 4H-1-benzothiopyran-4-ones (thiochromones) and related compounds were characterized by means of mass, NMR, and UV spectra in order to distinguish thiochromones from 2H-1-benzothiopyran-2-ones (thiocoumarins). In the mass spectra, the fragmentation due to the retro-Diels-Alder reaction directly from the molecular ion is the most useful for differentiation. In the NMR spectra, thiochromones show the ...