A study of the reactivity of HO2/O2- with unsaturated hydroperoxides/peroxides was carried out in a stopped-flow spectrophotometer equipped with an O2--generating plasma lamp. The results show that, in 80% aqueous ethanol solution containing either 0.05 M H2SO4 (for HO2 studies) or 0.005 M KOH (for O2- studies), these oxy-radicals do not react with oleic acid hydroperoxide, linoleic acid hydroperoxide, 1-hydroperoxy-2-cyclooctene, and tert-butyl allyl peroxide. These findings are discussed in the light of conflicting evidence concerning the reaction of HO2/O2- with organic hydroperoxides/peroxides.