The anti- and pro-oxidative effects of phenolic compounds and antioxidants were studied in two different in vitro model systems utilizing ethyl linoleate and 2'-deoxyguanosine (2'-dG) as oxidative substrates, and a Fenton reaction (H2O2, Fe2+) to initiate oxidation. Oxidation of the biomolecules in both model systems exhibited dose dependency. In the 2'-dG assay, oxidation was closely related to H2O2 generation, which occurred during autoxidation of the phenolics. Hydroxylating activity was greatly enhanced by Mn2+ and Cu2+, but not by Zn2+ or Co2+. Ethyl linoleate peroxidation was inhibited by low concentrations of catechol, quercitin, and instant coffee. However, peroxidation was promoted by high concentrations of the same compounds, probably by recycling of chelated inactive Fe3+ to the active Fe2+ state.