ABSTRACT: Binding to bovine serum albumin of monomeric (vescalagin and pedunculagin) and dimeric ellagitannins (roburin A, oenothein B and gemin A) was investigated by isothermal titration calorimetry and fluorescence spectroscopy, which indicated two types of binding sites. Stronger and more specific sites exhibited affinity constants, K₁, of 10⁴ to 10⁶ M^-1 and stoichiometries, n₁, of 2 to 13 and dominated at low tannin concentrations. Weaker and less-specific binding sites had K₂ -constants of 10³ to 10⁵ M^-1 and stoichiometries, n₂, of 16 to 30, and dominated at higher tannin concentrations. Binding to stronger sites appeared dependent on tannin flexibility and presence of free galloyl groups. Positive entropies for all but gemin A indicated that hydrophobic interactions dominated during complexation. This was supported by an exponential relationship between the affinity, K₁, and the modeled, hydrophobic accessible surface area and by a linear relationship between K₁ and the Stern-Volmer quenching constant, K_SV.