Syn carbonyl oxides generated in alkene ozonolysis have been implicated as sources of hydroxy radical (.OH) in the atmosphere. We report quantum chemical calculations at the B3LYP/6-31G(d,p), CBS-QB3, MPW1K/ 6-31+G(d,p), and CBS-APNO levels to characterize the reactivity of syn acetaldehyde oxide and its vinyl hydroperoxide isomer. The vinoxy radical formed upon vinyl hydroperoxide decomposition is converted to a chemically activated peroxy radical in the presence of O-2. All methods besides MPWIK predict that this species undergoes a 1,4-hydrogen shift with an activation barrier of similar to20 kcal/mol and then decomposes to yield .OH. RRKM/Master equation calculations predict that this unimolecular reaction of the peroxy radical will compete significantly with its collisional stabilization even up to 1 atm pressure. This chemistry can partly account for the .OD radicals recently observed in the ozonolysis of alkenes with vinylic deuteriums. The CBS-QB3 and CBS-APNO methods predict relative energies that agree to within 1 kcal/mol for most of the reactions considered in this study. The B3LYP/6-31 G(d,p) and MPW1K/6-31 +G(d,p) predictions are considerably less precise and often disagree with the model chemistry predictions.