Precise oxygen equilibrium curves of carp hemoglobin have been obtained in 0·1 m-phosphate from 10 to 25 °C. The equilibrium data were analyzed according to the stepwise oxygenation model of Adair (1925) to obtain the enthalpy change (ΔHi), entropy change (ΔSi) and free energy change (ΔGi) for the i (= 1, 2, 3, 4) individual oxygenation steps. The values of ΔHi are definitely non-uniform with dependencies on i and the pH of the medium. The co-operative effects in carp hemoglobin are due mainly to enthalpic contributions under the conditions studied here. The thermodynamic properties suggest a structural transition with pK ~8·5 as was also seen in other functional and spectroscopic measurements.