Cadmium adsorption on kaolinite-based clays in the absence and presence of humic acid was modeled with the aid of the FITEQL 3.2 computer program using a modified Langmuir approach for capacity calculations. Formation of surface–metal ion and surface–humate–metal ion complexes was assumed using the DLM approach. As Cd(II) adsorption was ionic strength-dependent, the adsorption experiments were carried out in solutions containing two different concentrations of an inert electrolyte (0.1 M and 0.005 M NaClO4). The surface sites responsible for the adsorption were assumed to be the permanent charges, ≡S1OH silanol groups and carboxyl groups having pK a values close to that of the silanol groups, and ≡S2OH aluminol groups and phenol groups with pK a values close to that of the aluminol groups, because the studied clays (partly composed of clay soil) contained organic carbon. Cd2+ ions were assumed to bind to the surface in the form of outer-sphere X2 2− Cd2+ and inner-sphere ≡SOCd+ monodentate complexes. When humic acid was added, Cd(II) adsorption was modeled using a multi-site binding model by the aid of FITEQL3.2. The fit between model and experimental values was excellent in each case. Since the stability of the ternary surface complexes in the presence of humic acid was higher than that of the corresponding binary surface–cadmium ion complexes, the adsorption vs. pH curves were much steeper (and distinctly S-shaped) compared to the tailed curves observed in binary clay–cadmium ion systems. The clay mineral in the presence of humic acid probably behaved more like a chelating ion-exchanger for heavy metal ions than as a simple inorganic ion exchanger.