Oxygen equilibrium curves of the extracellular hemoglobin from Lumbricus terrestris were determined under a variety of conditions. These data were characterized by (i) a rather small free energy of cooperativity (1.6-2.8 kcal/mol), (ii) a large and strongly pH-dependent Hill coefficient with a maximum value of 7.9, (iii) a high sensitivity of the upper asymptote of the Hill plot to pH, and (iv) a maximum association constant as large as that of the free beta subunit of human hemoglobin A. The effects of LiCl, KCl, NaCl, BaCl2, CaCl2, SrCl2, and MgCl2 on the oxygen equilibrium were measured. Cations, not Cl-, were found to control oxygen binding. Divalent cations have a larger effect on oxygen affinity than monovalent cations, and their effectiveness decreased in the order listed above within each valence class. These specific effects depend in part on ionic radius and cannot be explained in terms of ionic strength. The data indicate that the oxygenation-linked binding of a Ca2+ ion is accompanied by the release of two protons; the binding of a Na+ ion is associated with the release of one proton. These findings indicate that the oxygenation-linked cation-binding site contains two acid groups that do not readily dissociate their protons except when replaced by cations. Incubation at either pH 6.2 or 8.9 had no effect on subsequent measurements of oxygen equilibria at pH 7.8. The apparent heat of oxygenation was found to be -11.8, -7.3, and -9.3 kcal/mol at pH 9.0, 7.4, and 6.6, respectively. These differences indicate that proton-binding processes contribute to the heat of oxygenation.