The unimolecular decomposition of the nitrooxyalkyl radicals arising from the NO3-initiated reaction of isoprene is investigated. Density functional theory and ab initio molecular orbital calculations have been employed to determine the structures and energies of the transition states of decomposition of the nitrooxylalkyl radicals and the corresponding oxirane products. Geometry optimizations were performed using density functional theory at the B3LYP/6-31G(d,p) level and the single-point energies were computed using second-order Moller-Plesset perturbation theory and the coupled-cluster theory with single and double excitations including perturbative corrections for the triple excitations (CCSD(T)). At the CCSD(T)/6-31G(d) level of theory, the decomposition barriers range from 13 to 21 kcal mol-1, and the decomposition products (i.e., the separated oxirane and NO2) are 9 to 25 kcal mol-1 more stable than the nitrooxylalkyl radicals. The rate constants of decomposition of the nitrooxylalkyl radicals and the pressure-dependent oxirane yields have been calculated using the transition state theory and master equation formalism.