Nitroxides are cell-permeable stable radicals that protect biomolecules from oxidative damage in several ways. The mechanisms of protection studied to date include removal of superoxide radicals as SOD-mimics, oxidation of transition metal ions to preempt the Fenton reaction, and scavenging carbon-centered radicals. However, there is no agreement regarding the reaction of piperidine nitroxides with peroxyl radicals. The question of whether they can protect by scavenging peroxyl radicals is important because these radicals are formed in the presence of oxygen abundant in biological tissues. To further our understanding of the antioxidative behavior of piperidine nitroxides, we studied their effect on biochemical systems exposed to the water soluble radical initiator 2,2'-azobis (2-amidinopropane) hydrochloride (AAPH). AAPH thermally decomposes to yield tert-amidinopropane radicals (t-AP(*)) that readily react with oxygen to form peroxyl radicals (t-APOO(*)). It has recently been reported that piperidine nitroxides protect plasmid DNA from t-AP(*) though not from t-APOO(*). The present study was directed at the question of whether these nitroxides can protect biological systems from damage inflicted by peroxyl radicals. The reaction of nitroxides with AAPH-derived radicals was followed by cyclic voltammetry and electron paramagnetic resonance spectroscopy, whereas the accumulation of peroxide was iodometrically assayed. Assaying DNA damage in vitro, we demonstrate that piperidine nitroxides protect from both t-AP(*) and t-APOO(*). Similarly, nitroxides inhibit AAPH-induced enzyme inactivation. The results indicate that piperidine nitroxides protect the target molecule by reacting with and detoxifying peroxyl radicals.