The protonation enthalpies of some polycarboxylate anions (malonate, citrate, 1,2,3-propanetricarboxylate, and 1,2,3,4-butanetetracarboxylate) were studied calorimetrically at different ionic strengths, 1 ≤ I ≤ 5 mol kg^-1 , in NaCl aqueous solutions. Some potentiometric measurements were also made at 15 and 55 °C. Both calorimetric and potentiometric data (including previously reported data) were used to obtain complete sets of ΔG, ΔH, and TΔS values for the protonation of carboxylate anions. The dependence of ΔH on ionic strength was modeled by simple linear equations and using the Pitzer equations. Both ΔH values and their dependence on ionic strength follow regular trends as a function of the protonation steps, and general empirical relationships are proposed.