The surface complexation model which so far has been applied to the sorption of ions onto charge-bearing surfaces has been extended to the competitive adsorption of heavy metal salts and strong acids onto weakly basic anion exchanger. Considering local equilibria, logarithmic equilibrium parameters (generalized separation factors) can be deduced by measuring pH values and equilibrium concentrations. The generalized separation factors can be plotted as a function of the dimensionless loadings of the resin phase with metal salts and acids. By this means a set of two equilibrium constants is obtained for the uptake of each component. As found in previous publications about true ion exchange equilibria, it is the major advantage of the theoretical approach that these constants remain unchanged in multicomponent systems. Evaluation of experiments with two commercially available weakly basic anion exchanger products demonstrate the validity of the approach. By means of the parameters derived from simple systems the sorption equilibrium for systems with two heavy metals and systems with additional amounts of neutral salt can be well predicted.