The exchange of magnesium for calcium at 298 K on Wyoming bentonite was investigated in a SO mol m~3 perchlorate background maintained at pH 7. The charge fraction of Na+ in the aqueous solution phase was controlled at the values of 0.74 or 0.87 during the exchange experiments, resulting in average exchangeable sodium percentages (ESP) of either 16 or 36% on the clay. The Vanselow selectivity coefficient for Ca —• Mg exchange was equal to 1.0 and was independent of exchanger composition at both ESP values. This result implied that, under the conditions of the exchange experiments, the clay exhibited equal affinities for Ca2* and Mg2+ regardless of the quantity of adsorbed sodium present.