The ion-exchange equilibrium constants of the H+-Zn(II), H+-Cd(II) and H+-Fe(III) systems for the Strongly acidic cationic resins Dowex CM-15 and Dowex C650 have been determined. The results have been interpreted by means of the surface complex formation model, the three-parameter model and the Wilson model. A methodology for the prediction of the equilibrium constant values of the three-parameter and Wilson models from the values obtained using the surface complex formation model is given and discussed.