Inositol hexaphosphate (IHP) binding to aquomethemoglobin A (metHb) has been studied by how microcalorimetry and compared to IHP interactions with carbonmonoxyhemoglobin A (COHb) and deoxyhemoglobin A (deoxyHb). At 25 degrees C and pH 6.8 in 0.05 M NaPhos containing 0.1 M NaQ and, after correction for heats of proton extraction from buffer, MP binds to metHb, deoxyHb and COHb with enthalpies of -80.3, -104.2 and -105.9 kJ/mol IHP, respectively. None of the above processes exhibits large apparent heat capacity changes. The binding to metHb is strongly linked to proton uptake, 1.5 H+ being absorbed per MP bound at pH 6.8, with an enthalpic contribution of -21 kJ/mol Hf absorbed. Inorganic anions, including chloride, phosphate and perchlorate, bind to metHb with dissociation constants in the 0.1 M range and are released upon IHP binding. At high salt concentrations 2-4 Such ions are released with enthalpic contributions of +13 to +29 kJ/mol anion released. Thus observed heats of reaction represent a difference of large energetic terms, including not only intrinsic interaction of ligand with protein but also heats of proton extraction from buffer, proton uptake by protein and anion displacement by ligand, each of which can exceed 42 kJ/mol. These results are discussed in light of a possible allosteric transition accompanying MP binding to metHbA.