Tea extracts contain a number of phytochemicals including catechins and their derivatives (in green tea) or theaflavins along with thearubigins (in black tea). We evaluated the inhibitory effect of several different tea extracts (Darjeeling, Earl Grey, Kukicha, Oolong, and Sencha) in oxidation reaction of tyrosine via peroxynitrite. Peroxynitrite is a cytotoxic species generated by the reaction between superoxide and nitric oxide. We followed the footprint of peroxynitrite oxidation reaction by detection and quantitation of nitrotyrosine formation. A synergistic interaction between tea extracts and either ascorbic acid or trolox (an Ą-tocopherol analogue) was observed, i.e. the measured protective effect of tea extract mixed with ascorbic acid or trolox was greater than the effect of each inhibitor individually. Furthermore we provided evidence that this synergistic effect is the result of the regeneration of antioxidant, which is analogous to what is observed in the known interaction between ascorbic acid and vitamin E. In order to differentiate between hydrogen atom or electron transfer mechanism, we carried out a mechanistic study comparing tea extract inhibition activities and their measured antioxidant capacity in two different in vitro assays: Oxygen Radical Absorbance Capacity (ORAC) and Folin-Ciocalteu reagent (FCR). The result of this study may suggest possible enhanced therapeutic effect of tea extracts in the presence of vitamin C or vitamin E. Details of our study and a proposed mechanism will be presented.