Peroxy radicals with b, g double bonds which mainly originate in singlet oxygen attack to polyunsaturated fatty acids but also from the above mentioned rearrangement of autoxidation adducts, may undergo cyclisation reactions in competition with reactions leading to monohydroperoxides. The resulting epidioxides may suffer oxygen addition and abstraction of a hydrogen atom yielding hydroperoxide-epidioxides .

Peroxy radicals with isolated b, g double bonds are formed as intermediary products after autoxidation and photooxidation (reaction with singlet O₂) of unsaturated fatty acids having two or more double bonds. Fragmentation occurs when a hydroperoxide-epidioxide is heated, resulting in the formation of aldehydes and aldehyde-acids.

Peroxy radicals interact rapidly with antioxidants which may be present to give monohydroperoxides. Thus, it is not only the chain reaction which is inhibited by antioxidants, but also b-fragmentation and peroxy radical cyclisation.