The isomer pair HCSHSC and related cations have been studied by means of a highly accurate level of theory. For all the species investigated the near-equilibrium potential energy surface has been calculated using the coupled cluster method in conjunction with correlation consistent basis sets ranging in size from quadruple to sextuple zeta. After extrapolation to the complete basis set limit, additional corrections due to core-valence correlation and scalar relativistic effects have also been included. Consequently, the molecular and spectroscopic properties as well as the ionization potentials and dissociation energies have been predicted to high accuracy. Isomerization path and energy for both radical and cationic species have also been investigated. Finally, the anharmonic vibrational frequencies have been employed in order to obtain zero-point corrections to ionization potentials, dissociation energies, and isomerization barriers: IP0(HCS) = 7.57(4) eV and IP0(HSC) = 9.00(5) eV; D0(C-H) = 49.29(55) kcal/mol and D0(S-H) = 9.99(37) kcal/mol; deltaE0(HCSHSC) = 39.29(49) kcal/mol, and deltaE0(HCS+HSC+) = 72.24(75) kcal/mol.