The scavenging effects of tea catechins and their epimerized, acylated, and glucostylated derivatives on 1,1-diphenyl-2-picrythydrazyl (DPPH) radical were evaluated by electron spin resonance spectrometry. Tea catechins and their epimers were shown to have 50% radical scavenging ability in the concentration range of 1 to 3 microM. No significant differences were observed between the scavenging activities of tea catechins and their epimers, and, hence, the scavenging effects of catechins are not dependent on their sterical structure. The relationship between scavenging ability and the structure of tea catechins was also examined with acylated and glucosylated catechin derivatives. It is suggested that the galloyl moiety attached to flavan-3-ol at 3 position has a strong scavenging ability on the DPPH radical as well as the ortho-trihydroxyl group in the B ring, which elevates the radical scavenging efficiency above that of the ortho-dihydroxyl group; as has been recognized in other flavonoids such as flavones. The results obtained from the reactivity of tea catechins with the DPPH radical at different pHs suggest not only that the ortho-trihydroxyl group and the galloyl moiety contribute to maintaining the DPPH radical scavenging ability more effectively in a wide range of conditions from acidic to alkaline, but also that the radical scavenging efficiency of the ortho-dihydroxyls in the B ring is limited in neutral to alkaline regions. The difference between the scavenging abilities of the trihydroxyls (probably in the galloyl moiety) and the dihydroxyls can be explained in terms of redox potentials. It is concluded that the ortho-trihydroxyl group in the B ring and the galloyl moiety at 3 position of flavan-3-ol skeleton are the most important structural features for displaying an excellent scavenging ability on the DPPH radical.