The mechanism for the reaction of cytochrome c with t-butyl hydroperoxide and cumene hydroperoxide was investigated. ESR spin trapping studies using 5,5-dimethyl-1-pyrroline N-oxide were performed to demonstrate the presence of hydroperoxide-derived peroxyl, alkoxyl, and methyl radicals. Computer simulation of the experimental data obtained at various 5,5-dimethyl-1-pyrroline N-oxide concentrations was used to determine the relative contributions of each radical adduct to each composite ESR spectrum. From these analyses, it was concluded that the alkoxyl radical of the hydroperoxide was the initial radical produced, presumably by homolytic scission of the O-O bond by ferric cytochrome c. This was in contrast to a previous ESR study that proposed a heterolytic peroxidase-type mechanism for the reaction of cytochrome c with organic hydroperoxides. Methyl radicals were produced from the beta-scission of the alkoxyl radical. The peroxyl radicals are shown to be secondary products formed from the reaction of oxygen with the methyl radical to produce the methyl peroxyl radical. In separate experiments, visible absorption spectroscopy revealed that the heme center was destroyed during the reaction. Both the heme destruction and production of radical adducts were inhibited by cyanide, presumably due to the formation of a cyanoheme complex.