Most plant-derived polyphenols exhibit strong antioxidant potentials, established by various assay procedures. With pulse radiolysis experiments, absolute scavenging rate constants can be obtained with a variety of oxidizing radicals which allow further structure-activity correlations and, combined with EPR spectroscopy, detailed insight into the mechanisms governing these antioxidant reactions. The most striking difference occurs between regular flavonoids and both condensed and hydrolyzable tannins. The tannins are considered superior antioxidants as their eventual oxidation may lead to oligomerization via phenolic coupling and enlargement of the number of reactive sites, a reaction which has never been observed with the flavonoids themselves.