Methods of tandem mass spectrometry have been applied to the isomeric ethyl thioformates, HC(=S)OC2H5 (I), and HC(=O)SC2H5 (II), searching for the elusive thionformic acid HC(=S)OH in both ionized and neutral forms. Given the complete analogy of either the high- or low-energy collisional activation spectra and also the analogy of the ion-molecule reactions involving the [M-C2H4](+.) ions of both I and II, it is proposed that the radical cations actually generated have in both cases the thiohydroxy-hydroxy-carbene structure [HS-C-OH](+.). This common structure is proposed on the basis of an observation of competitive losses of SH and OH radicals upon collisional activation and MS/MS/MS experiments. The [M-C2H4-OH](+) produced by collisional activation of the [M-C2H4](+.) ions are expected to have a C-S-H, rather than H-C-S connectivity. Ab initio MO calculations at the UMP2/6-31G(d,p) level provide indirect support for the observation of the carbene.