The complexation equilibrium of metals (copper, lead, and zinc) by using a chelating ion-exchange resin (Duolite ES-346) is analyzed. Two potentiometric titrations of the resin which allow evaluation of the exchange capacity and the pK values are carried out. Two equilibrium models (mass action type model and surface complex formation model) are proposed, and their parameters are estimated by fitting model equations to data from batch experiments. The degradation of the amidoxime groups in acidic media and a possible way to minimize that problem is discussed.