The trans rotamer of thioformic acid, t-HC(O)SH, was detected toward the galactic center source G+0.693–0.027 by Rodríguez-Almeida and co-workers, as reported in 2021. The IRAM 30m and the Yebes 40m telescopes were employed to make the observations, in which nine a-type transitions were detected. Trans-thioformic acid has also been tentatively detected toward the massive protostellar core G31.41+031 by García de la Concepción and co-workers. The cis form was not detected by either effort. Wang and co-workers identified a possible formation path to t-HC(O)SH on interstellar ice, while Molpeceres and co-workers characterized the destruction of formic and thioformic acid via H-abstraction reactions.

In addition to trans and cis rotamers—more properly anti and syn forms, since the rotation is around a single bond instead of a double bond—thioformic acid has two isomeric forms that are close in energy. The sulfur in the detected and most stable form of thioformic acid, HC(O)SH, is in the form an –SH thiol, the sulfur version of the –OH hydroxyl radical in alcohols. In the other form of thioformic acid, HC(S)OH, the sulfur is in the form of a =S thione, the sulfur version of a =O ketone or aldehyde. Lignell and co-workers characterized the vibrational spectra of these compounds, as well as dithioformic acid, HC(S)SH. At least two other less stable isomers are known, sulfine, H₂CSO, and HCSOH, which is a carbene stabilized by dative π bonding between the C and S atoms (not a C≡S triple bond as indicated by Schreiner et al.). Structures of t-HC(S)OH, H₂CSO, and HCSOH are shown below.

t-HC(S)OH	H2CSO	HCSOH
(thione isomer)	(hypervalent sulfur)	(carbene)