Exchange isotherms were prepared for Na+ in Na+—»Ca2+ and Na+^Mg2+ exchange reactions, and for Mg2+ in Ca2+—»Mg2+ exchange reactions, at 298 K on Wyoming bentonite suspended in a 0.05M perchlorate background. These isotherms were essentially congruent with the appropriate thermodynamic nonpreference exchange isotherms. It was concluded from this fact that there is essentially no difference in the affinity of montmorillonite clay for Ca2+ vs. Mg2+. However, exchange isotherms prepared for Na+ in Na(I) — Ca(II) and Na(I) — Mg(II) exchange, as well as published ones for Mg(II) in Ca(II) — Mg(II) exchange on montmorillonite suspended in a 0.05M chloride background consistently indicate a slight preference on the clay for Ca(II) over Mg(II). This preference for Ca(II) is concluded to be the result of the formation of CaCl+ complexes, which are more stable thermodynamically and have a greater affinity for the clay than MgCl+ complexes.