The ephemeral Criegee intermediate, first postulated over 70 years ago, has only recently been isolated in the gas phase. The sulfur analogue of this canonical zwitterion, thioformaldehyde S-sulfide, has eluded similar analysis; however, argon matrix isolation has been achieved (Angew. Chem., Int. Ed. 2001, 40, 393-396). Here thioformaldehyde S-sulfide and its valence isomer dithiirane are examined with high-level coupled-cluster methods, including the minimum-energy pathway for interconversion. Relative enthalpies calculated from extrapolated energies at the complete basis set limit of the full CCSDTQ method are reported. Isomerization from thioformaldehyde S-sulfide to the lower-lying dithiirane (-7.2 kcal mol(-1)) is predicted to include a 27.0 kcal mol(-1) barrier. Harmonic and anharmonic vibrational frequencies are also predicted using second-order vibrational perturbation theory. These results should aid in future gas-phase identification.