The [H,S,Cl] potential-energy surface has been investigated at the self-consistent ®eld (SCF), complete active space self-consistent ®eld (CASSCF), secondorder Mùller±Plesset, coupled-cluster single-double and perturbative triple excitation, [CCSD(T)]/6-31G(d,p), 6-31G(2df,2pd), and correlation-consistent polarized valence triple zeta (cc-pVTZ) levels of theory. CCSD(T)/ cc-pVTZ results predict a very stable HSCl species, an isomer HClS, 51.84 kcal/mol higher in energy, and a transition state 57.68 kcal/mol above HSCl. Independent of the level of theory, results with the smaller 6-31G(d,p) basis set turned out to be poor, especially for HClS. Vibrational analysis indicates that both species can be easily dierentiated if isolated. Bonding dierences between these molecules are illustrated by contour plots of valence orbitals. Viewed classically, bonding in HClS involves a dative bond. Transition-state rate constants, and equilibrium constants for the HSCl 6 HClS isomerization have been estimated for various temperatures (200±1000 K). At 298.15 K, the forward rate is predicted to be 7.95´10 )29 s )1 , and the equilibrium constant to be 2.31´10 )38 . Tunneling corrections vary from 1.57 at 298.15 K to 1.05 at 1000 K. Activation energies have been obtained by a two-points linear ®t to the Arrhenius equation.