Selectivity coefficients for Pb2+, Cd2+, and Ca2+ exchange adsorption with montmorillonite, illite, and kaolinite clays were determined over a wide range of clay surface concentration and found constant. The average selectivity coefficients describing the ion distributions for montmorillonite, illite, and kaolinite, respectively, were 0.60, 0.44, 0.34 for Pb-Ca exchange, 1.04, 1.01, 0.89 for Cd-Ca exchange and 0.58, 0.56, 0.31 for Pb-Cd exchange. Whereas Cd2+ may compete more or less on an even basis with Ca2+ for clay adsorption sites, the adsorption of Pb2+ would be favored by a factor of 2 or 3 over Ca2+.