In this work, a convenient test of antioxidant activity was developed, with BChE-contaminated HSA as the target of AAPH-induced oxidation and its esterase activity as the marker of protein integrity or degradation. The method is relatively simple, of low cost, and convenient to use. Its application to natural polyphenols showed that quercetin (1), verbascoside (2), chlorogenic acid (3), caffeic acid (4),1,3,6,7-tetrahydroxyxanthone (5), and mangiferin (6), are good antioxidants (IC50 < 9 muM). 1,5-Dihydroxy-3-methoxyxanthone (7), flemichin D (8), and cordigone (9) showed modest activities (ca. 50 muM < IC50 < 350 muM), whereas danthrone (10) was inactive. Complementary experiments with two of the more active antioxidants, namely quercetin (1) and chlorogenic acid (3) showed that both antioxidants were better radical scavengers than chain-breaking antioxidants. The relative adiabatic oxidation potential (DeltaH(ox)), the relative H-bond dissociation energy (DeltaH(abs)), and the first oxidation potential measured by cyclic voltammetry were found to be related to the radical-scavenging activity of these antioxidants.