The thermodynamic parameters of the magnesium ion induced self-association of calf brain tubulin in pH 7.0, 0.01 M phosphate buffer containing 10(-4) M GTP, were determined from sedimentation velocity experiments. This reaction proceeds by an isodesmic mechanism terminated by the highly favored formation of a closed ring shaped polymer of degree of association 26 +/- 4. Analysis of the variation of the apparent dimerization constant in the isodesmic mechanism s,ows that this self-association is characterized by positive enthalpy, entropy, heat capacity, and molar volume changes, as well as the binding of one additional magnesium ion, which is probably not involved as a bridge between the protein molecules. The addition of the last monomeric subunit has a free energy which is about three times that of dimer formation. Under the conditions of these experiments, tubulin binds 48 +/- 5 magnesium ions with a free energy of --2.8 kcal/mol.