Thermodynamic parameters for the interaction of two acrylic-maleic copolymers (MW = 3 and 70 kDa) with H+, Na+, Mg2+ and Ca2+ were obtained from potentiometric and calorimetric measurements. Apparent protonation constants were expressed as a function of a (dissociation degree) using the three parameter model proposed by Ho¨gfeldt and the formation constants of Na+, Mg2+ and Ca2+ complexes were calculated from the differences in protonation constants in the different ionic media. The salt effects on protonation were interpreted in terms of both activity coeffi- cient variations and cation complex formation. Enthalpy changes for protonation, obtained by direct calorimetry (always quite low, the stabilization being mainly of entropic nature), were also expressed as a function of a and I using empirical equations. The formation constants obtained for alkali and alkaline earth metal complexes are quite low for Na+ species (depending on a and on ionic strength, we always found K < 10 L mol 1 ), and fairly high for Mg2+ and Ca2+ species, ML and ML2