The antioxidant capacity of a series of structurally related flavonoids is quantified by the amount of ABTS [2,2'-azinobis-(3-ethylbenzthiazoline-6-sulfonate)] radical anions (ABTS(.-)) that is able to react with the flavonoid and expressed as the Trolox equivalent antioxidant capacity (TEAC). To evaluate the predictive value of the TEAC, the antioxidant activity of this series of flavonoids was also assessed in other in vitro assays, measuring the effect on hydroxyl scavenging, lipid peroxidation and doxorubicin-induced toxicity as typical scavenging or damage assays. The flavonoids tested were mono HER, di HER, tri HER, tetra HER and tri HEQ, differing in the number of aromatic hydroxyl groups. It was found that these compounds showed both a fast and slow scavenging effect in the TEAC assay and therefore the TEAC at 10 s ('fast' TEAC) and 6 min ('total' TEAC) was determined. Both this 'total' and 'fast' TEAC are negatively correlated with hydroxyl radical scavenging. The 'total' TEAC showed a better correlation than the 'fast' TEAC with the inhibition of lipid peroxidation and the protection against doxorubicin-induced toxicity. This indicates that beside the fast reaction of scavengers with the ABTS radical, also the slow reaction should be taken into consideration. It is concluded that the antioxidant capacity, assessed with the modified TEAC assay, can be useful to predict the in vivo antioxidant effect in a series of structurally related compounds.