The adsorption of copper(II) onto goethite is qualitatively different from that of other metal-mineral systems. At pH 5.0 the adsorption isotherms are sigmoid rather than hyperbolic, and the corresponding graphs of C/N versus C (where C is concentration and N adsorption density) have distinct minima. At pH 5.5 the isotherms have the more usual shape. The experimental data can be fitted by a model In which monomeric CuOH+ and dimeric Cu-2(OH)(2)(2+) compete for surface sites, the dimer adsorbing more strongly to the surface. As the concentration increases the proportion of the dimeric species increases, which leads to enhanced adsorption. At pH 5.5 adsorption of the monomer is sufficiently strong that normal behavior is observed at low concentrations, before there is sufficient dimer to compete effectively for sites. Data are presented for adsorption of Cu(II) to goethite at pH 5.0 and pH 5.5, at temperatures between 25 and 70 degrees C. Adsorption is endothermic for both monomer and dimer. Enthalpies of adsorption range from 50 to 91 kJ mol(-1), and entropies from 240 to 470 J K-1 mol(-1).