Polyphenol is a plant secondary metabolite containing more than one hydroxy group at the benzene ring. They are known to exhibit a range of various bioactivities, including strong antioxidant activity. The exhibiting of antioxidant activity, namely the oxidation reaction of polyphenols, produces unstable intermediates and drastically changes their chemical structure. This reaction occurs not only under biological conditions but also under food processing and storage conditions. In this review, some examples of polyphenol oxidation reactions elucidated using model reaction systems will be introduced. In particular, the chemical structure changes of polyphenols and the oxidation mechanisms inferred from these changes will be described.