Plants synthesize a diversity of tannin structures but little is known about whether these different types have different oxidative activities in herbivores. Oxidative activities of hydrolyzable and condensed tannins were compared at pH 10 with two methods: EPR spectrometry was used to quantify semiquinone radicals in anoxic conditions and a spectrophotometric assay was used to measure the rate of browning of phenolics oxidized in ambient oxygen conditions. A little-studied group of hydrolyzable tannins (ellagitannins) contained the most active tannins examined, forming high concentrations of semiquinone radicals and browning at the highest rates. On average, galloyl glucoses and high-molecular-weight gallotannins had intermediate to low oxidative activities. Condensed tannins generally formed low levels of semiquinone radicals and browned most slowly. The results suggest that ellagitannin-rich plants have active oxidative defenses against herbivores, such as caterpillars, whereas the opposite may hold true for plants that contain predominantly condensed tannins or high-molecular-weight gallotannins.