The protonation constants of four different chelating resins containing: iminodiacetate (Ligandex-I resin), diethylenetriamine-tetracetate (DTTA resin), ethylenediamine (ED resin) and ethylenediamine-triacetate (Ligandex-E resin) functional groups were calculated from their pa-metric titration curves obtained at different ionic strengths at 25 degrees C. For the evaluation of the titration curves both the traditional Gibbs-Donnan based calculation method and the recently suggested, so called, three-parameter model have been applied. The applicability of these models are compared and discussed on the base of the goodness of fit of the experimental and theoretical titration curves. The standard deviation in pH ranges from 0.01 to 0.24 and within this Limits the two methods give identical results. The comparison of the protonation constants of the resins and their corresponding monomers leads to the conclusion that the studied ligands follow a rather ideal behavior in their immobilized state.