Unimolecular pathways for the dissociation of thiylperoxyl have been investigated computationally. Gaussian-2 (G2) theory was employed to calculate the thermochemistry, and the kinetics of SH + O-2 --> HSOO are analyzed by RRKM theory. Under atmospheric conditions the rate constant is close to the low-pressure limit of k(0) approximate to 9.2 x 10(-34)(T/298 K)(-1.69) molecule(-2) cm(6) s(-1) for T = 200-400 K. By comparison to other tropospheric oxidation reactions of thiyl radicals, addition to oxygen yields the shortest lifetime, although the low G2 S-0 bond dissociation enthalpy implies a small equilibrium constant and means that HSOO formation is likely to be important only below 298 K. Singlet and triplet SOO have been characterized, and Delta H-f,H-298((SOO)-S-1) is predicted to be 185.5 kT mol(-1) The results are discussed in the context of atmospheric chemistry.