We have examined the antioxidant activity of one class of polyphenolic compounds in green beans: two novel flavonol glycosides (quercetin 3-O-[xylosyl(1→2)]-rhamnosyl (1→6)-glucoside and the corresponding kaempferol analogue), quercetin 3-O-glucuronide, kaempferol 3-O-glucuronide, quercetin 3-O-rutinoside and kaempferol 3-O-rutinoside. The Trolox equivalent antioxidant capacity (TEAC) and inhibition of iron/ascorbate-induced lipid peroxidation of phosphatidyl choline vesicles were measured. In the aqueous phase TEAC assay, the glucuronide and rutinoside of quercetin were good antioxidants, but not as effective as the quercetin aglycone. TEAC values for the glucuronide and other glycosides of kaempferol were much lower than the corresponding quercetin species but similar to that of the kaempferol aglycone. Quercetin 3-O-glucuronide and quercetin 3-O-rutinoside were both potent inhibitors of lipid peroxidation, in contrast to the those of kaempferol. The compounds described herein demonstrate the antioxidant activity of the flavonols present in green beans and indicate the effect on antioxidant activity of sugar substitutions in the phenolic C ring.