Thioxy-hydroxy-carbene (HS-C-OR) and its radical cation have been generated in the gas phase upon dissociative ionization of ethyl thioformates and characterized by various MS-RIS-MS experiments. In the present study, their energies and unimolecular rearrangements have further been determined with the aid of ab initio molecular orbital calculations. Potential energy surfaces for both neutral [CH2OS] and ionized [CH2OS](.+) species constructed at the QCISD(T)/6-311 + + G(d,p)//(U)MP2/6-31(d,p) level confirm that in both states, the carbene form is kinetically stable. While HS-C-OH is 179 kJ mol(-1) less stable than the thiol acid HC(O)SH, [HS-C-OH](.+) becomes even more stable than [HC(O)SH](.+) and lies at 77 kJ mol-l higher in energy than the thion acid [HC(S)OH](.+), the most stable ion isomer. While it is not involved in the unimolecular chemistry of neutral thioformic acids, carbene plays a key role in that of ionized isomers. Some thermochemical parameters of HS-C-OH are estimated as follows: heat of formation Delta H-f(0) = 63 kJ mol(-1) at 0 K and 57 kJ mol(-1) at 298 K; ionization energy E-i = 8.6 eV, and single-triplet gap Delta ES-T = - 156 kJ mol(-1) in favour of the singlet state. For thioformic acid, its heat of formation is evaluated to be Delta H-f(0), 298[HC(O)SH] = - 124 kJ mol(-1) and proton affinity PA[HC(O)SH] = 773 kJ mol(-1), with errors of +/- 10 kJ mol(-1).