In this paper, the thermodynamic properties of the complex species of the Gantrez (Trade Mark product) copolymers of different molecular weights (AN169, S97, and S95) with different divalent (Ca2+, Mg2+, Sn2+, and Zn2+) metal cations in NaCl aqueous solutions at different ionic strengths and temperatures were studied by a potentiometric technique. Investigations were carried out in wide ranges of metal to ligand molar concentration ratios up to basic pH values. For the simple metal-Gantrez systems, fairly similar speciation models were obtained, independent of the experimental conditions; the main differences are due to the formation of a ternary MLOH species (M = metal ion, L = Gantrez ligand) or to the final pH value of the titrations, determined by the formation in some cases (i.e., for Sn2+ investigations) of a sparingly soluble species. The stability trend of the species was: Sn2+ ≫ Zn2+ > Ca2+ ≈ Mg2+. For Gantrez S95, the interactions with Zn2+ and Sn2+ were also investigated in a solution containin...