Abstract: 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 potentiometric 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 pH+ + qCu2+ +≡SOH⇔(H+)p(Cu2+)q(≡SOH)(p+2q)+βp,q(int)s=[HpCuq(≡SOH)(p+2q)][H+]p[Cu2+]q[≡SOH] 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 ≡TOH − H+ ⇔ ≡TO− logβ−1,0(int)S = −5.77 (± 0.07). 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.