Binary cation-exchange processes (Na-K, Na-Mg, and Na-Ca) have been investigated at 25 degrees C in samples of bentonite from Cabo de Gata (Almería, SE Spain). Exchange isotherms and Vanselow selectivity coefficients indicate that the preference of this bentonite for K(+), Ca(2+), and Mg(2+) ions is greater than that for the Na(+) ion. The change in (i)(j)K(V) as a function of the equivalent fraction of cation M, E(M), is interpreted on the basis of the presence of different types of adsorption/exchange sites (tetrahedral, octahedral, and edge sites). For the Na/K exchange, (K)(Na)K(V) exhibits a steep decrease for E(K)<0.25-0.3. This is interpreted as due to preferential exchange at the tetrahedral sites. However, the opposite behavior is observed in the case of Ca and Mg, with a noticeable increase of (i)(Na)K(V) for E(Ca)(E(M(g)))>0.6-0.8. The compared selectivities of Ca and Mg for this smectite are nearly equal. As far as the exchanger is concerned, the Na-Ca and Na-Mg exchange is nearly ideal, in the raoultian sense. Thermodynamic exchange constants and the corresponding Gibbs free energies of reaction have been computed from (i)(j)K(V) coefficients. This allow us to quantify the exchanger's affinity sequence, which appears to be Na(+)<K(+)<Mg(2+)<Ca(2+). Copyright 2001 Academic Press.