The exchange of magnesium for calcium at 298 K on a montmorillonitic soil separate (principally clay and silt) was investigated in a 50 mol m~3 perchlorate background at pH 5,6, and 7. The charge fraction of sodium in the aqueous solution phase was controlled at 0.70 or 0.85 during the exchange experiments, resulting in average exchangeable sodium percentages ranging from 5 to 25% on the soil. Exchange isotherms for magnesium indicated some preference on the soil for calcium over magnesium. This preference was attributed primarily to the presence of organic matter associated with the soil separate. However, the exchange isotherms showed no effect of pH in the range between 5 and 7, and no effect of exchangeable sodium percentage in the range between 0 and 25%.