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Dietary flavonoids can be detected in plasma as protein-bound conjugates. Flavonoids-protein interaction is expected to modulate the bioavailability of flavonoids. In this work, the binding flavonoid isomers (galangin, baicalein, apigenin, and genistein; MW=270.25) and B-ring hydroxylation flavonols (galangin, kaempferol, quercetin, and myricetin, which share the same structure on the A and C rings but have 0, 1, 2, and 3 moieties of -OH on the B-ring, respectively) to protein were investigated by fluorescence quenching method. The apparent binding constants (Ka) of were flavonoid isomers determined as: flavones (106-107 L mol-1)>isoflavone≈flavonol (105 L mol-1). For B-ring hydroxylation flavonols, the binding affinity increased with increasing number of hydroxyl groups on the B-ring. The binding constants (Ka) were determined as follows: myricetin>quercetin>kaempferol>galangin. [ABSTRACT FROM AUTHOR]