The amount of adsorption of sulfate and Cu(II) from single- and binary-adsorbate systems on goethite were measured. All experiments were carried out with and without supporting electrolyte (0.01 M NaNO3) as a function of equilibrium pH (2similar to7), adsorbate concentration (0.21similar to1.57 mM), and temperature (15similar to35degreesC). At a given equilibrium pH, it was shown that the adsorption isotherms of single sulfate, rather than Cu(II), could be well described by the Langmuir equation. The isosteric enthalpy of sulfate adsorption was also evaluated. In contrast to a single-adsorbate system, the adsorption of Cu(II) was enhanced and that of sulfate was inhibited in binary adsorbate systems under the conditions studied. Finally, the changes in zeta potentials of the goethite suspensions with solution pH before and after adsorption were measured, which could be macroscopically related to the amount of sulfate adsorption even in binary-adsorbate systems.