AbstractThe exchange isotherms of the ion pairs Ca‐Na, Ca‐K, Na‐K, Ca‐H and Na‐H on the clay mineral montmorillonite were determined.It was found that the selectivity coefficient is not constant along a given isotherm but changes with the relative fractions of the adsorbed ions. This variation is attributed to the structural properties of the mineral, namely, tactoid formation in the Ca state, and the presence of different exchange sites affecting the energy of