Spin trapping coupled with electron paramagnetic resonance spectroscopy is the most direct method for detection of very low concentrations of free radicals, and it has been intensively used to detect superoxide or alkylperoxyl radicals in biological systems, using cyclic nitrones as spin traps. The half-life time of the ensuing spin adducts depends dramatically on the spin trap structure; however, their mechanism of decay has never been definitely established. We investigated at the MRMP2/ CASSCF (MultiReference second-order Møller-Plesset perturbation theory/Complete Active Set Self Consistent Field) level of theory the mechanism of decay of methylperoxyl and tert-butylperoxyl spin adducts formed with various cyclic nitrones. We showed that no transition state can be located for the O─O homolytic cleavage, which yields an intermediate biradical with the following sequence • O─N─C─O • . Then, homolytic cleavage of the N─C bond yields a