The successful fitting of the Wilson equation to solid phase activity coefficients for binary aqueous systems makes possible the construction of three-component graphs for the ternary systems: (Na-Rb-Cs) Wyoming montmorillonite, (Na-Rb-Cs) Chambers montmorillonite, and (Na-K-Cs) attapulgite. Given the composition of the liquid (solid) phase, one can predict the composition of the solid (liquid) phase. The results showed (i) that the ion exchanger in ternary systems still tended to prefer the counter-ion with smaller equivalent volume, (ii) that attapulgite ion-exchange properties were similar to montmorillonites and could be regarded as intermediate between Wyoming and Chambers, and (iii) that soil which contained montmorillonite with low CEC might be more susceptible to becoming sodic than would soil which contained montmorillonites with high CEC.