Competitive complexation with respect to the binary-metal ion combinations Cu(II)-Zn(II), Cu(II)-Pb(II), and Pb(II)-Zn(II) were studied at the goethite (alpha-FeOOH)-water interface (25 degrees C, I = 0.1 M NaNO3). In addition to potentiometric titrations and batch adsorption experiments, an in situ voltammetric technique was utilized. Sorption studies were performed within the ranges 3.5 less than or equal to pH less than or equal to 8.5, 10(-6) less than or equal to [M(II)](tot) less than or equal to 10(-3) M, and with solid concentrations between 0.09 and 9 g dm(-3). The results obtained (metal uptake and proton release curves) were compared with corresponding predicted values, based upon previously determined thermodynamic surface complexation models for the different single-metal ion goethite systems. The surface complexation modeling was based upon the constant capacitance concept, The results showed that the Cu(II) and Zn(II) surface complexation submodels were combinatory in all two-metal systems studied, both at 10(-3) M and at 10(-6) M concentrations of the metals. For Pb(II) a somewhat stronger adsorption to goethite than predicted by the previously determined submodel was observed in all two-metal systems studied, These deviations could be corrected for by a moderate adjustment of the equilibrium constant for the most acidic surface complex =FeOHPb2+. It can also be concluded that the in situ voltammetric technique can be applied to measure trace concentrations of at least two metal ions simultaneously occurring in goethite suspensions under varying experimental conditions with good accuracy.