The kinetics and the mechanism of the thermal decomposition of F-4S and subsequent reactions have been studied. The rate constant for the initiation reaction HiS + M -products (1) was determined by a shock tube-infrared emission spectroscopy at temperatures 2740-3570 K to be it| = io~1044±031 exp[-(268.6±18.4)kJ mo\~]/RT] cm3 molecule"' s~', which is about one-fifth to one-tenth of the recent results reported by Woiki and Roth (J. Phys. Chem. 1994, 98, 12958) and Olschewski et al. (J. Phys. Chem. 1994, 95, 12964). An ab initia (MRCI+Q) calculation suggested that a spin-forbidden product channel (-*S(3P) + H2) is energetically favorable compared to a H-S bond fission channel; that is, the singlet-triplet intersystem crossing occurs at an energy lower than the dissociation threshold for HS + H by about 17 kJ mol~'. The present rate constant for reaction 1 could be well reproduced by an unimolecular decomposition theory with the calculated energy for the crossing and with a reasonable collision parameter, βc. The rate constants for important subsequent reactions, S(3P) + H2 -* products (3) and S(3P) + F-4S - products (4), were also determined by a laser photolysis-shock tube-atomic resonance absorption spectrometry method: Jt3 = 10-9.5s±o.i6 exp[-(82.5±4.0) kJ mo\~l/RT] (1050-1660 K) cm3 molecule-' s~\ and Jt4 = 10-9-85±a'7 exp[-(30.9±4.1) kJ mor'/β7] (1050-1540 K) cm3 molecule"1 s~'. The ARAS measurement of H atoms revealed that the main products for reaction 3 are HS + H at pressures below 2 atm.