The dynamics of the enthalpy and volume changes related to the photo-dissociation of oxygen from human and bovine oxyhemoglobin are investigated by nanosecond time-resolved photoacoustic calorimetry (PAC). The values of enthalpy and volume change associated with the above process are DeltaH = 37.8 +/- 3 kcal/mol, DeltaV = 5.0 +/- 1 ml/mol for human HbO(2); and DeltaH = 35.7 +/- 3.5 kcal/ mol, DeltaV = 4.8 +/- 1 ml/mol for bovine HbO(2), respectively. A possible explanation for the similar values between both human and bovine oxyhemoglobin is proposed. In addition, the PAC results for human HbO(2) and HbCO are compared and discussed. (C) 2004 Elsevier Inc. All rights reserved.