All the exchange isotherms of Na+ or Ca2+ in vermiculite for K+, Rb+ or Cs+ show two kinds of exchanges sites: the first corresponds to sites located in phlogopite-like interlamellar spaces and the second to sites located in smectite-like interlamellar spaces. During the exchange, three mineralogical phases are formed: Na- or Ca-vermiculite; K-, Rb- or Cs-vermiculite and a mixed-layer phase. Far infrared absorption bands of K+, Rb+ and Cs+ located in the phlogopite-like interlamellar spaces are at 72, 59 and 53 cm–1, respectively. These frequencies are characteristic of a trioctahedral mixed-layer phase. Selectivity is greater for cations with a low hydration energy (K+, Rb+, Cs+) than for cations with a high hydration energy (Na+, Ca2+). On the other hand, for cations with a low hydration energy, structural parameters such as the tetrahedral rotation angle, α, have to be taken into account.