Two isoflavones in vivo metabolites, genistein-7-O-β-D-glucuronic acid (G7G) and daidzein-7-O-β-D-glucuronic acid (D7G) were synthesised chemically. The ability of these metabolites to scavenge an organic radical was measured by the trolox equivalent antioxidant capacity (TEAC) assay, while their reducing ability was measured by the ferric reducing antioxidant power (FRAP) assay. The TEAC and FRAP values of G7G were 45 and 51% of that of genistein, while those of D7G were 52 and 77% of that of daidzein, respectively. A direct assessment of G7G and D7G antioxidant activity by their ability to delay copper(II)-mediated lipid oxidation of human LDL showed that these metabolites retained the ability to prevent oxidation in the lipid phase, but activity was diminished compared to their corresponding aglycones. However, G7G and D7G also decreased the rate of lipid oxidation to 53 and 86% of control without isoflavones, respectively, indicating a continuous exchange of antioxidants between the aqueous environment and the LDL lipid phase during the whole oxidation period.