Calorimetric measurements at 25 degrees of the differential heat of CO binding by hemoglobin trout I have been examined together with the CO binding isotherms for the protein at 4 degrees and 20 degrees. Simultaneous treatment of these data sets by a statistically rigorous technique permits evaluation of all the thermodynamic parameters for both the Adair and the Monod, Wyman, Changeux (MWC) models. The results show the details of the unusual temperature dependent cooperativity which this hemoglobin exhibits. In the Adair formalism the increasingly favorable free energy change for successive steps of ligand binding are nearly linearly paralleled by increasingly negative enthalpy changes for these steps. This causes the enhanced cooperativity observed as the temperature is decreased. For the MWC case, lowering the temperature increases the stability of the unligated T state relative to the unligated R state since the enthalpy of the T leads to R transition is 29.4 kcal mol-1. Simultaneously, the favorability of ligating R forms relative to T is enhanced since R form ligation is 14.1 kcal (mol CO)-1 more exothermic than that of T. The balance between these opposing effects is to increase ligand binding cooperativity at low temperatures. The predicted temperature dependence of the Hill coefficient for the MWC and Adair models is identical at low and intermediate temperatures, but, interestingly, would show a strong divergence at high temperatures where negative cooperativity is suggested for the Adair case and positive cooperativity for the MWC case.