Potassium-Ca and Na-Ca exchange isotherms were obtained for two kaolinite samples and two kaolinitic soils (Hutton and Avalon forms). The Gapon, Vanselow, and Gaines-Thomas corrected selectivity coefficients were usually greater for K-Ca exchange than for Na-Ca exchange, as was expected. The exchange isotherms and selectivity coefficients suggested a preference for K relative to Ca on all four exchangers. The contribution of micaceous impurities to the observed high affinity for K was taken into consideration, by reasoning that exchange sites with high affinity for K resulting from these impurities are first saturated with K [i.e., at low exchangeable potassium percentage (EPP) values]. It is suggested that the high charge density of kaolinite enhances K dehydration, resulting in a high affinity of kaolinite per se for K.