The reaction behavior of condensed tannins and related polyphenols with ammonia (NH 3 ) water was examined. NH 3 water treatment of pyrogallol, gallic acid, and epigallocatechin, which have a pyrogallol nucleus, caused regioselective amination in the presence of molecular oxygen (O 2 ) to form 2-aminoresorcinol, 4-amino-3,5-dihydroxybenzoic acid and 4′- C -amino-epigallocatechin, respectively. In contrast, other polyphenols, such as catechol, protocatechuic acid, and epicatechin, which have a catechol nucleus, did not undergo amination even in the presence of O 2 . Under nitrogen atmosphere (in the absence of O 2 ), epimerization at the C-2 position took place during NH 3 treatment of epigallocatechin, in addition to amination. The same epimerization took place during NH 3 treatment of epicatechin regardless of the presence or absence of O 2 . By NH 3 treatment, pyrogallol-type B-rings of condensed tannin from Acacia mearnsii underwent regioselective amination to yield 4′-amino-3′,5′-dihydroxybenzene-type B-rings in the presence of O 2 , whereas catechol-type B-rings of condensed tannin from quebracho showed little amination regardless of the presence or absence of O 2 . From these results, it was concluded that amination of pyrogallol nucleus by NH 3 treatment should proceed via oxidation and reduction, and therefore this reaction would be attributed to the oxidation property of condensed tannins and related polyphenols.