The reaction of C(P-3) with H2S to HCS/HSC isomers is investigated with high level ab initio methods. Besides reaction energies to form HCS/HSC, the energetics and properties of different singlet/triplet H2CS reaction intermediates are studied. The combination of ab initio theory and crossed molecular beams experiments leads to the conclusion that mainly HCS is formed under single collision conditions and that the reaction is indirect and proceeds through a thiohydroxycarbene intermediate.