The potential energy surface for the decomposition of singlet HXCS (X = H and F) has been explored by B3LYP and CCSD(T) calculations. Five different types of reaction are proposed: (A) 1,1-HX elimination, (B) 1,2-H shift, (C) 1,2-X shift, (D) H . and XCS . radical formation, and (E) X . and HCS . radical formation. These results show interesting trends for the HXCS isomers. Our theoretical investigations suggest that the doubly bonded species HXC=S is the lowest energy structure among the isomers from both kinetic and thermodynamic viewpoints. We also report theoretical predictions of molecular parameters and vibrational IR spectra of the monohalogen substituted thioformaldehyde, which should be useful for future experimental observations.