Plant polyphenols act as antioxidants mainly by trapping reactive oxygen species and by regenerating endogenous membrane-bound α-tocopherol (vitamin E). In both processes polyphenols are oxidized. Hence, knowledge of the oxidation mechanisms of polyphenols is important for an understanding of their antioxidant activity at the molecular level. This work focuses on anthocyanins (pigments) and flavanols (tannins), two important classes of polyphenols which are both relatively abundant in human diet. The oxidation of the 3′,4′,7-trihydroxyflavylium ion (1) and catechin (2), respectively taken as models for anthocyanins and tannins, has been investigated. From kinetic data and partial product analysis, the mechanisms for the reactions of 1 and 2 with sodium periodate and DPPH, a H atom-abstracting radical, are proposed. Both polyphenols are shown to form o-quinone intermediates upon H atom abstraction and subsequent radical disproportionation. In the case of 2, the quinone and a second molecule of antioxidant quickly couple to form dimers. By contrast, 1 is extensively degraded into coumarins by repeating sequences of oxidation–solvent addition, which consume several equivalents of oxidants. In aqueous solutions, 1 is typically a mixture of coloured and colourless forms. The latter (chalcones) are also shown to take part in the antioxidant activity.