The caffeoyl conjugates of prenylhydroquinone glucoside and of quinic acid, either in the carboxyl-free or carboxymethyl forms, isolated from Phagnalon rupestre (Asteraceae), showed inhibitory activity on lipid peroxidation induced by Fe 2+/ascorbate and by CCl4/NADPH in rat liver microsomes, with IC50 values ranging from 3 to 11 microM. After having demonstrated their effect on the xanthine oxidase-regulated superoxide production, the active compounds were tested for the direct inhibition of this enzyme. Methylated dicaffeoylquinic conjugates competitively inhibited the enzyme and the highest potency was obtained for the 4,5-diester, with an IC50 value of 3.6 microM, nearly ten times lower than that of the 3,5-analogue. In conclusion, the presence of the caffeoyl moiety is essential for both the antiperoxidative and radical scavenging activities, and the methylation of the quinic carboxyl group enhances the potency on xanthine oxidase inhibitory activity.