Flavanols and oligomeric proantho-cyanidins were isolated from oak bark ( QUERCUS PETRAEA) and characterized by acid hydrolysis, gel permeation chromatography, haemanalysis, and complete as well as partial thiolytic degradation. The proantho-cyanidins had an average polymerization degree of 6.1 and a procyanidin: prodelphinidin ratio of 6:4. The prevailing units of the oligomeric chains were (+)-catechin, (-)-epicatechin, and (+)-gallocatechin, minor units were (-)-epicatechin 3- O-gallate and (-)-epigallocatechin 3- O-gallate. The bark contains both hydrolyzable and condensed tannins. Although only 23% of the water-soluble oak bark tannins consisted of oligomeric proanthocyanidins, these contributed with 55% to the astringency of the total tannin fraction.