Dietary polyphenolics in fruits, vegetables, wines, spices and herbal medicines have beneficial antioxidant, anti-inflammatory and anticancer effects. However, we have observed that dietary polyphenolics with phenol rings were metabolized by peroxidase to form prooxidant phenoxyl radicals which, in some cases were sufficiently reactive to cooxidize GSH or NADH accompanied by extensive oxygen uptake and reactive oxygen species formation. The order of catalytic effectiveness found for oxygen activation when polyphenolics were metabolized by peroxidase in the presence of GSH was phloretin>phloridzin>4,2'-dihydroxy chalcone>p-coumaric acid>naringenin>apigenin>curcumin>resveratrol>isoliquiritigenin>capsaicin>kaempferol. Ascorbate was also cooxidized by the phenoxyl radicals but without oxygen activation. Polyphenolics with catechol rings also cooxidized ascorbate, likely mediated by semiquinone radicals. The order of catalytic effectiveness found for ascorbate cooxidation was fisetin luteolin, quercetin, >eriodictyol, caffeic acid, nordihydroguaiaretic acid>catechin>taxifolin, catechol. NADH was stoichiometrically oxidized without oxygen uptake which, suggests that o-quinone metabolites were responsible. GSH was not cooxidized and GSH conjugates were formed, likely mediated by the o-quinone metabolites. Incubation of hepatocytes with dietary polyphenolics containing phenol rings was found to partially oxidize hepatocyte GSH to GSSG while polyphenolics with a catechol ring were found to deplete GSH through formation of GSH conjugates. Dietary polyphenolics with phenol rings also oxidized human erythrocyte oxyhemoglobin and caused erythrocyte hemolysis more readily than polyphenolics with catechol rings. It is concluded that polyphenolics containing a phenol ring are generally more prooxidant than polyphenolics containing a catechol ring.