Oxidative desulfurization of thioamides is known to occur in vivo. For metabolic studies of thioamides, the need for the corresponding amide as a reference material focussed our attention on the chemical conversion. Synthetic methods for preparation of amides from thioamides have numerous deficiencies. During studies to verify the structures of certain antiviral thioamides, the electron-ionization mass spectra were determined. A common fragmentation, loss of 33 amu from the molecular radical cation, was observed. Exact mass measurements showed that this fragmentation corresponds to the loss of SH radical. The facile loss of SH directed our attention to a search for a suitable reagent which could function as an oxygen-exchange agent. We have found methyl sulfoxide to behave as such an agent. Thus, thioamides are quickly and quantitatively transformed into their corresponding amides by treatment with hydrochloric acid in methyl sulfoxide at ambient temperature. Quantitative conversion is rapidly achieved, typically in less than one minute, at room temperature and no further processing of the reaction mixture is required for instrumental analysis. Unequivocal proof of the integrity of the starting thioamide and the product amide moieties was obtained by gas phase Fourier-transformed infrared spectroscopy.