The association constant for insulin dimerization calculated from concentration-dependent circular dichroic (CD) spectra of insulin, K12 = 7.5 X 10(5) M-1, is used along with other association constants (K24, K46, and K26) in an attempt to decipher the complex association behavior in solution and in crystal of this protein hormone. The free-energy change associated with dimerization, -RT ln K12, is -8.01 kcal mol-1, a value which is used to test a semiquantitative thermodynamic model of the process based in part on the X-ray crystallographic data of insulin. By delineation of the hydrophobic core on the surface of insulin, which is implicated in the dimer formation, the free energy of association, delta G degrees assoc, is estimated as -8.27 kcal mol-1 by using the thermodynamic parameters of Némethy & Scheraga [Némethy, G., & Scheraga, H. A. (1962) J Phys. Chem. 66, 1773-1789] and as -10.27 kcal mol-1 by using the values of Nozaki & Tanford [Nozaki, Y., & Tanford, C. (1971) J. Biol. Chem. 246, 2211-2217]. The role of hydrophobic bonding in the dimerization of insulin is discussed, and the calculated values of free energy of association are compared with the experimental value. The importance of this thermodynamic model is delineated in regard to both hormone-hormone and hormone-receptor interactions.