The binding reaction of betaxolol (BET) with bovine serum albumin (BSA) in aqueous buffer solution has been investigated using isothermal titration calorimetry (ITC) and circular dichroism (CD) spectroscopy. The thermodynamic results indicate that there were two classes of binding sites on each BSA molecule for BET molecules. The changes of standard Gibbs free energy (Delta G(1)degrees and Delta G(2)degrees)are almost the same when the drug molecules bind to the first and the second classes of sites. However, the changes of standard enthalpy (Delta H-1 degrees and Delta H-2 degrees) are -38.35 +/- 0.50 and 18.06 +/- 0.03 kJ mol(-1), respectively. The first class of binding is an enthalpy driven process while the second class of binding is an entropy driven one. The results of spectroscopic experiment were applied to investigate the structure of the BSA-BET complex and to understand the thermodynamic data. (C) 2010 Elsevier B.V. All rights reserved.