A sample of an ombrotrophic peat was washed thoroughly with nitric acid to release bound cations, and potentiometric titrations were carried out with the untreated peat and the acid-washed peat at five different ionic strengths of between 0.01 and 0.3. M. The experimental titration curves were used to obtain the absolute charge-pH curves for the acid-washed peat and relative charge-pH curves for the untreated peat (i.e. for the latter only the surface charge due to ionization of acid groups, as the value of the initial charge was not known because of the presence of exchangeable cations). The effect of ionic strength on proton binding was the same for the acid-washed peat and the untreated peat, as indicated by the similar spacing between the charge curves for both types of samples. The charge curves for the acid-washed and untreated peat corresponded after correction of the charge of the untreated peat, considering that the initial charge is equal to the charge on the exchangeable cations. The NICA-Donnan model, in which the Donnan volume varied with ionic strength, provided an excellent fit to the experimental data for the acid-washed peat and enabled the values of the intrinsic proton binding parameters to be obtained. Good simulation of the experimental charge curves for the untreated peat was obtained from the parameters of the acid-washed peat, by use of the same values of the Donnan volume, and including the charge due to the exchangeable cations. The parameters of the acid-washed peat therefore provide a realistic representation of the observed behavior of the untreated peat, and enable prediction of the behavior of peat in the natural environment. © 2011 Elsevier B.V.