The potentiometric titration curves of aqueous solutions of polygalacturonic acid were determined in the presence of either tetraalkylammonium or alkaline ions in order to explain the effects of the size and the nature of the counterion on the degree of dissociation. In the evaluation of potentiometric curves, the approach proposed by Strauss was used and successive ionization constants of appropriately chosen oligomeric subunits of the polymer molecule were determined. The partition functions and Gibbs free energies of ionization per mole of polygalacturonic acid at the ambient chemical potential of hydrogen ions, and the Gibbs free energies per mole of polyacid related to the hydrogen ion in its standard state, were calculated and are given as a function of hydrogen ion activity or of the degree of ionization. From the above functions the differences in the counterion specificities were observed and are discussed on the basis of the formation of contact ion pairs between an alkaline ion and the carboxylate group. In addition, the values of the entropy and enthalpy changes per mole of repeating unit (hydrogen ion at standard state potential) accompanying the stepwise dissociation reactions were calculated on the basis of the temperature dependence of the stepwise dissociation constants via the Gibbs-Helmholtz relation, and the thermodynamic data were correlated with the structural properties of the investigated polymer solute.