In this work, the NO3 addition to alkenes has been studied using ab initio quantum chemistry methods. In addition to the commonly accepted NO3 radical addition to one of the C=C carbon atoms in alkenes, a symmetric transition state corresponding to the cycloaddition of the nitrate radical to the double bond has been identified, leading to the formation of a particularly stable NO3-alkene cyclic radical. Consideration of the cyclic adduct is necessary to explain the formation of carbonyl products derived from the cleavage of the C C bond in the absence of oxygen. Yet, the possibility of its formation from the nitroalkyl open adduct radical R1R2C(ONO2)CR3R4. is hampered by the presence of a high energy barrier leading from the open to the cyclic adduct. The proposed cycloaddition channel is analogous to the Criegee mechanism for the cycloaddition of ozone to double bonds.