The surface complexation model has been applied to the description of the sorption equilibrium of selenium species onto weakly basic anion exchangers. 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. By this means a set of two equilibrium constants is obtained for the uptake of each component. The major advantage of this theoretical approach is that these constants remain unchanged in multicomponent systems. Evaluation of experiments with two commercially available weakly basic anion exchangers demonstrate the validity of the approach. The equilibrium of binary (SeO(3)(2-), NO(3)(-) or SeO(4)(2-), NO(3)(-)), ternary (SeO(3)(2-) /Cl(-)/NO(3)(-), SeO(4)(2-)/NO(3)(-) and SeO(3)(2-)/SeO(4)(2-)/NO(3)(-)) and quaternary systems (SeO(3)(2-)/SeO(4)(2-)/Cl(-)/NO(3)(-)) can be well correlated. (C) 2009 Elsevier Ltd. All rights reserved.