Investigators in the U.S. report, "Functional ingredients of grape seeds include several flavonoids with a phenolic nature such as monomeric flavanols (catechin and epicatechin), dimeric, trimeric and polymeric procyanidins, and phenolic acids (gallic acid and ellagic acid).
“These flavonoids have been reported to exhibit antioxidant activity in vivo and in vitro in a number of studies."