The kinetics and the mechanism of the thermal decomposition of H2S and subsequent reactions have been studied. The rate constant for the initiation reaction H2S + M --> products (1) was determined by a shock tube-infrared emission spectroscopy at temperatures 2740-3570 K to be k(1) = 10(-10.44+/-0.31) exp[-(268.6+/-18.4)kJ mol(-1)/RT] cm(3) molecule(-1) s(-1), 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, 98, 12964). An nb initio (MRCI+Q) calculation suggested that a spin-forbidden product channel (-->S(P-3) + H-2) 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(-1). 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, beta(c). The rate constants for important subsequent reactions, S(P-3) + H-2 --> products (3) and S(P-3) + H2S --> products (4), were also determined by a laser photolysis-shock tube-atomic resonance absorption spectrometry method : k(3) = 10(-9.58+/-0.16) exp[-(82.5+/-4.0) kJ mol(-1)/RT] (1050-1660 K) cm(3) molecule(-1) s(-1), and k(4) = 10(-9.86+/-0.17) exp[-(30.9+/-4.1) kJ mol(-1)/RT] (1050-1540 K) cm(3) molecule(-1) s(-1). The ARAS measurement of H atoms revealed that the main products for reaction 3 are HS + H at pressures below 2 atm.