Peroxynitrite is a powerful biological oxidant generated in vivo from the reaction between nitric oxide and superoxide. Decomposition of peroxynitrite (ONOO ) occurs through protonation to peroxynitrous acid (ONOOH). Thus, at pH 11-12, peroxynitrite is stable for days; whereas, at pH 7, it has a half-life of approximately 1 sec. The decomposition mechanism has long been associated with free radical generation, yet the formation of free hydroxyl radical during the decomposition of peroxynitrous acid has been shown to be thermodynamically unlikely. Spin trapping using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) has yielded some evidence for the DMPO-OH adduct. another report shows the formation of DMPOX, a further oxidation product of DMPO. We characterized the reaction between ONOO and a novel spin trap, DEPMO. We were unable to detect the DEPMPO-OH adduct. However, when glutathione (GSH) was present, we observed the formation of the DEPMPO-SG adduct. The DEPMPO-SG signal was not affected by the presence of hydroxyl radical scavengers indicating, as has been reported previously, that the mechanism is a direct reaction between ONOO and GSH and does not involve the HO-like activity of ONOO . We are currently investigating the EPR spectra of DEPMPO adducts formed in systems containing acidified nitrite and hydrogen peroxide.