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 H2S in Ar could be analyzed. First-order rate constants of k/[Ar] = 4.0 x 10(14) exp(-33000 K/T) cm(3) mol(-1) s(-1) 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 --> H-2 + S(P-3) + M with a rate constant k(1) which is given by k(1) = k/2, i.e., k(1)/[Ar] = 2.0 x 10(14) exp(-33000 WT) cm(3) mol(-1) s(-1) between 1800 and 3300 K. The measured rate constants are analyzed in terms of unimolecular rate theory, and a threshold energy of E(0) = 295 kJ mol(-1) is derived which agrees with the endothermicity of the above reaction.