Rapid scavenging of the model stable radical cation, ABTS(.+) has been applied to screen for the antioxidant activity of flavonoids, The reaction follows two distinct phases. For compounds with a monophenolic B-ring there is a rapid initial phase of reduction of ABTS(.+) within 0.1 s with no further change in the subsequent 2.9 s, In contrast, compounds with a catechol-containing B ring follow a fast initial scavenging phase with a slow secondary phase. Flavonoids with an unsubstituted B ring do not react within this time scale. The findings suggest that the structure of the B ring is the primary determinant of the antioxidant activity of flavonoids when studied through fast reaction kinetics.