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The thermal decomposition of H2S was studied in shock waves by monitoring UV absorption signals in the range 210-330 nm. Following the decay of H2S at 215 nm, experiments with 200-5000 ppm of H S in Ar could be analyzed. First-order rate constants of W[Ar] = 4.0 x 1014 exp(-33000 WT) cm3 mol-' s-' were obtained between 1800 and 3300 K. As these results complement observations on the formation of S atoms (Woiki, D.; Roth, P. J . Phys. Chem., preceding paper in this issue), the thermal dissociation of H2S is proven to proceed by the following spin-forbidden elimination reaction: H2S + M -H2 + S(3P) + M with a rate constant k~ which is given by k~ = W2, i.e., kll[Ar] = 2.0 x 1014 exp(-33000 WT) cm3 mol-' s-' between 1800 and 3300 K. The measured rate constants are analyzed in terms of unimolecular rate theory, and a threshold energy of EO = 295 kJ mol-' is derived which agrees with the endothermicity of the above reaction.

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