SCF-gradient and configuration-interaction (CI) methods have been used in a study of the lowest singlet and triplet states of various isomers on the C 2H2S energy surface. The main conclusions drawn from the calculations are the following: the singlet isomer with the lowest energy is thioketene with mercaptoacetylene 17.4 kcal/mol higher in energy. The much debated antiaromatic isomer thiirene is found to be 37.5 kcal/mol above thioketene and is found to be considerably lower in energy than thioformylmethylene carbene (∼32 kcal/mol) or the 1,3-diradical (∼22 kcal/mol) formed by scission of a CS bond. Consequently, thiirene has some stability towards thermal rearrangement by this route. © 1983 American Institute of Physics.