The interaction of H+- and Cu2+-ions with Ca-montmorillonite was investigated in 0.1 mol/dm3 solutions of Ca(CIO4)2 at 298.2 K by Potentiometrie titrations using both glass electrodes (for H+) and ion specific electrodes (for Cu2+ ). The experimental data were interpreted on the basis of the surface complexation model. The calculations were performed with the least-squares program FITEQL (Westall, 1982) using the constant capacitance approximation. The best fit was obtained with a set of equilibria of the general form and the constants log β1,0(int)S = 8.16 (± 0.04), log β-1,0(int)S = −8.71 (± 0.08), log β0,1(int)S = 5.87 (± 0.06), log β−1,1(int)S = −0.57 (± 0.12), log β−2,1(int)S = −6.76 (± 0.02). An appropriate modeling of the H+ adsorption data requires the introduction of a second surface group ≡ TOH with the acidity constant In addition, the ion exchange equilibria Ca2+ − Cu2+ and Ca2+ − H+ had to be taken into account. Arguments are presented to identify the groups ≡ SOH and ≡ TOH as surface aluminol groups =Al(OH)(H2O) and surface silanol groups ≡ Si-OH, respectively.