Abstract: The changes in the enthalpy and the conformational volume in photolyses of carboxy-hemoglobins (HbCO) of mammals, including human, porcine, bovine, equine and rabbit, are measured by the time-resolved photoacoustic calorimetry. A pulsed laser with width of 8 ns is used to excite photolysis reactions of HbCO and thus acoustic pulse signals related to the reaction processes are generated. The acooustic signals are detected by a PZT transducer with the resonant frequency of 1.5 MHz. Considering the effective response time of the experimental system and the different lifetimes for the reaction processes in the photolyses of HbCO, the detected results are found to correspond to the tertiary relaxation of the carboxy-hemoglobins with the lifetime of 800 ns. The results show that the enthalpy and the conformational volume changes for different mammals are different. Finally, the results are analyzed and discussed.