The kinetics and thermochemistry of the decomposition pathways for 1,3-disilacyclobutane (1,3-DSCB) in the gas phase were studied using the second-order Moller-Plesset (MP2) perturbation theory and coupled cluster methods with single, double, and perturbative triple excitations (CCSD(T)). The reactions examined include 2 + 2 cycloreversion to form two silenes by either a concerted or a stepwise mechanism, 1,1-, 1,2-, and 1,3-H-2 elimination, and the ring-opening initiated by 1,2-H shift to form an open-chain 1,3-disilabut-1-ylidene, which undergoes further decomposition to produce two pairs of silene/silylene species. The structures of the transition states for the concerted and the stepwise 2 + 2 cycloreversion pathways are found to resemble closely those reported for the head-to-tail and head-to-head dimerization, respectively. Comparison of the activation barriers demonstrates unambiguously that the stepwise cycloreversion (Delta H-0(double dagger) = 66.1 kcal/mol) is favored over the concerted one (Delta H-0(double dagger) = 77.3 kcal/mol). A new pathway was established from the 1,4-diradical intermediate in the stepwise cycloreversion to form 1-silylmethylsilene via 1,3-H shift. The concerted 1,1-H-2 elimination is shown to have the lowest activation barrier of all H-2 elimination reactions. Overall, the 1,2-H shift in 1,3-DSCB with concerted ring-opening to form 1,3-disilabut-1-ylidene is the most kinetically and thermodynamically favorable decomposition pathway, both at 0 and 298 K.