State-of-the-art, calibrated ab initio quantum chemical calculations have been performed to obtain accurate structures, energies, and vibrational frequencies of the lowest singlet states of HNS and NSH, which are closed-shell states. Enthalpies of formation (0 K) of these states of HNS and NSH are predicted to be in the ranges 53-60 kcal mol(-1) and 75-82 kcal mol(-1), and are likely to lie toward the lower limits. The transition state connecting these two isomers has been located and is predicted to lie 68 kcal mol(-1) above HNS, 44 kcal mol(-1) above NSH, and about 1.5 kcal mol(-1) below the H + NS asymptote. Calculations have also been performed on the lowest energy triplet states ((3)A ") of HNS and NSH, and these demonstrate that the closed-shell singlet states are indeed the ground states.