For accurate interpretation of cation binding to natural organic matter, the proton binding behaviour of both solid and dissolved natural organic matter must first be established. In the present study, potentiometric titrations of samples of humin and humic acid extracted from a peat soil were performed at different ionic strengths. Humic acid (HA) samples in solution (dissolved humic acid, DHA) and in suspension (aggregated humic acid, AHA) were titrated. The corresponding charge curves were analysed with the NICA-Donnan model and the results were compared with those previously obtained for the peat soil. Good reproduction of the DHA charge curves was obtained, and in the case of the AHA, the exact forms of the charge curves were not reproduced at pH < 6 because of the conformational changes and dissolution of the sample that took place throughout the titration. The peat and humin displayed similar proton binding behaviour, which was different to that of the humic acid. With the Donnan approach, the value of parameter b, that relates the Donnan volume to the ionic strength, was the same for peat and humin and less than that of the humic acid, for both the dissolved samples and those in suspension, indicating that the effect of ionic strength on the charge is greater in the peat and humin than in the HA. The ratio between the contents of phenolic groups and of carboxylic groups was greater in peat and humin than in humic acid. The model used revealed that the acid-base behaviour of the peat is closer to that of the humin than to that of the humic acid.