In this study, the calorimetric titration technique was used to determine the protonation enthalpy of two reference humic acids and polyacrylic acid. First, we obtained the apparent protonation constant of two kinds of humic acid purchased from IHSS (International Humic Substances Society) and polyacrylic acid by potentiometric titration. Second, we obtained the protonation enthalpy of them by calorimetric titration. The protonation enthalpy of humic acid was affected by pH and the ionic strength of bulk solution. From the comparison of Delta H between humic acid and polyacrylic acid, it was concluded that the pH dependence of DH is attributed to the heterogeneity of humic acid. And DH of phenolic hydroxyl group in humic acid is strongly influenced by the electric double layer of humic acid's surface. This is considered to be a reason of the ionic strength dependence of DH. On the other hand, Gibbs free energy of the protonation of humic acid is affected by the electrostatic attraction with the progress of dissociation of functional groups such as carboxyl group and phenolic hydroxyl group. Consequently, the thermodynamics of the protonation of humic acid is affected by the polyelectrolyte effect and the heterogeneity. (C) 2014 Elsevier Ltd. All rights reserved.