The relative hydrophobicity and interaction of flavonoids with artificial membranes using vesicles was studied. At the same degree of hydroxylation, flavones were slightly more hydrophobic than flavanones. Flavonoids possess a hydrophobic character and are weak acids. For this reason, their uncoupling efficiency of the membrane potential was studied using cytochrome c oxidase vesicles. With emphasis on naringenin, it was shown that flavonoids affect both the transmembrane potential difference (V) and the transmembrane pH difference (V). Flavones were slightly more effective in uncoupling the membrane potential than flavanones; the 7OH group seems to play an important role. Hydroxylation of the exocyclic phenyl group decreased the uncoupling efficiency for all flavonoids studied. The flavonol quercitin exhibited hardly any uncoupling activity. Glycosylation abolished all uncoupling activity. The affinity of flavonoids for vesicle membranes was also studied using the fluorescence quenching of the membrane probe diphenylhexatriene. Flavonols exhibited a substantially higher affinity for liposomes than flavanones. This difference in affinity is assumed to be caused by the far more planar configuration of the flavonols in comparison with the tilted configuration of flavanones. Due to this planar configuration, it seems reasonable to assume that flavonols could more easily intercalate into the organised structures of the phospholipids within the vesicle membranes than flavanones. It is concluded that, in vivo, hardly any uncoupling activity of flavonoids can be anticipated. However, the quercitin plasma concentration in vivo can be such that, based on the affinity study, part of this flavonol could be associated with biological membranes to function there as, for example, an antioxidant.