Spin trapping using 5,5-dimethyl-1-pyrroline Noxide (DMPO) has been used to detect and distinguish free radicals in samples of Greek extra virgin olive oils. A number of the samples examined immediately after the addition of the spin trap showed a spontaneous complex electron paramagnetic resonance (EPR) signal. The majority of DMPO-radical adducts formed (80-90%) represented peroxyl and alkoxyl radical adducts. Similar spectra were recorded when DMPO was added in oxidized triolein and then treated with Fe 2+ , Fe 3+ , or Cu 2+ or when EPR-silent olive oil samples were treated with these metallic ions. Metal ion-catalyzed decomposition of triolein hydroperoxides, as recorded by EPR signal intensity, increased with increasing metal ion concentration in the micromolar range. The relative concentration of alkoxyl-DMPO adducts increased with increasing Fe 2+ or Fe 3+ concentration, whereas that of peroxyl-DMPO species decreased. In contrast, the relative concentrations of alkoxyl and peroxyl species produced by Cu 2+ were similar over the whole metal concentration range examined. Exposure of EPR-silent virgin olive oil or oxidized triolein to ultraviolet light in the presence of DMPO resulted in the detection of a three-line spectrum characterized by wide line widths.Paper no. J9852 in JAOCS 78, 1121-1125 (November 2001).KEY WORDS: Copper, 5,5-dimethyl-1-pyrroline N-oxide (DMPO), electron paramagnetic resonance spectroscopy (EPR), free radicals, iron, olive oil, spin trapping.It has long been known that oxidative rancidity is the main deteriorative change of olive oil during storage and that it is due to the oxidation of unsaturated fatty acids and the subsequent formation of compounds possessing an unpleasant taste and odor (1). This general oxidation process affecting the stability of vegetable oils is often called autoxidation and involves a free radical mechanism. It is assumed that hydroperoxide groups attach to the carbon atom of unsaturated fatty compounds, and subsequently, the breakdown of hydroperoxides gives a chain reaction of autoxidation (2,3). Olive oil oxidation is affected by a number of factors, such as oxygen, temperature, presence of metals and chromophores, light, and ionizing radiation, whereas the resistance of virgin olive oil to oxidation is related to the high levels of monounsaturated triacylglycerols and the presence of natural antioxidants (1). Transition metals, especially iron and copper, are known contaminants of virgin olive oil and may act as pro-oxidant factors because they catalyze both the generation of free radicals and the decomposition of hydroperoxides (1-3). The oxidation of unsaturated fats is also accelerated by exposure to light, and direct photo-oxidation is due to free radicals produced by ultraviolet (UV) irradiation, which catalyzes the decomposition of hydroperoxides and other oxygen complexes of unsaturated lipids (2,3).Of the available methods for the detection of free radicals, only spin trapping offers the opportunity to simultaneously measure and distinguish amon...