To determine the role of surfactant hydroperoxides on the oxidative stability of fatty acids, the oxidation of methyl linoleate micelles and salmon oil-in-water emulsions was measured as a function of varying Tween 20 hydroperoxide concentrations. Increasing Tween 20 hydroperoxide concentrations from 3.5 to 14.7 micromol hydroperoxide/g Tween 20 decreased the lag phase of headspace hexanal formation but did not increase the total amount of hexanal formed in methyl linoleate/Tween 20 micelles. In the micelle system, Fe(2+) decreased the lag phase of hexanal formation but increased total hexanal concentrations only in micelles with the highest Tween 20 hydroperoxide concentrations (14.7 micromol hydroperoxide/g surfactant). Increasing Tween 20 surfactant hydroperoxide concentrations also increased the oxidation of salmon oil-in-water emulsions as determined by lipid hydroperoxides and headspace propanal. In both the micelle and emulsion systems, the prooxidant effect of Fe(2+) decreased with increasing Tween 20 hydroperoxide concentrations. These data show that surfactant hydroperoxides such as those in Tween 20 could decrease the oxidative stability of lipids in food emulsions.