Binary and ternary cation‐exchange reactions involving NH + 4 , K + , and Ca 2+ on vermiculite and hydroxy‐Al interlayered vermiculite (HIV) were investigated. In the case of vermiculite, NH 4 ‐Ca exchange was nearly indistinguishable from the nonpreference isotherm while HIV exhibited preference for NH + 4 The presence of K + had significant suppressing impact on the apparent NH + 4 selectivity in NH 4 ‐Ca exchange in HIV most likely because of retardation of Ca 2+ diffusion in the interlayer. For vermiculite, the presence of K + in the NH 4 ‐Ca system induced apparent high affinity sites for NH + 4 most likely because of interlayer collapse. The overall study demonstrated that cation‐exchange selectivity coefficients in vermiculite and HIV are dependent on number and type of cations present.