A systematic theoretical study has been performed on the low pressure thermal decomposition pathways of t-BuS(O)St-Bu using the CCSD(T)/cc-pV(D+d)Z,//B3LYP/6-311++G(2d,2p), CCSD(T)/cc-pV(D+d)Z.//PBEPBE/6-311++G(2d,2p), and G3B3 level of theories. Rate constants for the unimolecular decomposition pathways are calculated using Rice-Ramsperger-Kassel Marcus (RRKM) theory. On the basis of the experimental observation and theoretical predictions, the pyrolysis channels are considered as primary and secondary pyrolysis reactions. The primary decomposition via a five-membered transition state leads to the formation of tert-butanethiosulfoxylic acid (t-BuSSOH) and 2-methylpropene (C4H8) almost exclusively having low-pressure limit rate constant k(1)(0) = 4.67 x 10(-6)T(-4.67) exp(-11.64 kcal mol(-1)/RT) cm(3) mol(-1) s(-1) (T = 500-800 K). The primary decomposition via a six-membered transition state is also identified, and that leads to the tert-butanethiosulfinic acid t-BuS(OH)S, which is the branched chain isomer of t-BuSSOH. The formation of t-BuSSOH is thermodynamically as well as kinetically favorable over t-BuS(OH)S formation, and therefore the second product could not be found experimentally. Furthermore, calculation on secondary pyrolysis pathways involving the decomposition of t-BuSSOH leads to the formation of 1-oxatrisulfane (trans-HSSOH and cis-HSSOH) and their branched isomer S(SH)OH. These three secondary product formation rates are competitive, but thermodynamics do not favor the formation of the branched isomer. Among the secondary pyrolysis products, trans-HSSOH is the most stable one, and its formation rate constant at low pressure is calculated to be k(3)(0) = 5.49 x 10(28)T(-10.710) exp(-36.22 kcal mol(-1)/RT) cm(3) mol(-1) s(-1) (T = 800-1500 K). Finally, the secondary pyrolysis pathway from less stable product t-BuS(OH)S is also predicted, and that leads to trans-HSSOH and cis-HSSOH products with almost equal rates. A bond-order analysis using Wiberg bond indexes obtained by natural bond orbital (NBO) calculation predicts that the primary and secondary pyrolysis of t-BuS(0)St-Bu occur via El-like mechanism.