When the oxygen binding isotherms of human, bovine and fallow deer (Dama-Dama) hemoglobins are measured at different temperatures either by optical or calorimetric techniques, analyses according to the Adair's formalism show that at least one of the intermediate steps of ligation has a positive enthalpy change, i.e., absorbs rather than emitting heat, indicating that it is entropy rather than enthalpy driven (Bucci, E., et al. 1991. Biochemistry. 30:3195-3199; Bucci, E., et al. 1993. Biochemistry. 32:3519- 3526; Johnson, C., et al. 1992. Biochemistry. 31:10074-10082; Johnson, C., et al. 1995. Biophys. Chem. 59:107-117). This phenomenon is confirmed in systems in which the β82 lysines of human hemoglobin are covalently cross-linked by acylation with dicarboxylic acids of increasing length, namely the fumaryl (four-carbon-long), adipoyl (six-carbon-long), and sebacoyl (10-carbon-long) residues. Consistently in all of the systems here reported, the enthalpy excursions are masked by compensatory entropy changes, which keep the free energy of ligand binding constant for the first three steps of oxygenation. Furthermore, the cooperativity index and the overall oxygen affinity seem to be correlated to the positive enthalpy excursions of the intermediate steps of ligation. Fumaryl-Hb (hemoglobin cross-linked with a fumaryl residue, four carbons) with the lowest absorption of heat has the highest affinity and lowest cooperativity index. Adipoyl-Hb (hemoglobin cross-linked with an adipoyl residue, six carbons) has the highest absorption of heat and the highest cooperativity index. It appears that nonuniform heat release by the intermediates of oxygenation is part of the allosteric phenomena in hemoglobin systems. There is not enough information that would allow assigning these phenomena to the interplay of the various conformations described for hemoglobin besides the classic T (Fermi et al. 1984. J. Mol. Biol. 175:159-174) and R (Shanaan. 1983. J. Mol. Biol. 171:31-59), as listed at the end of the Discussion. The possibility cannot be excluded that entropy-driven steps characterize new conformational transitions still to be described.