A synthetic dioctahedral vermiculite was saturated with Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr and Ba, and examined by X-ray diffraction and infrared spectroscopy under various dehydration conditions. Cations which were larger than the ditrigonal hole (Cs, Rb, Ba) increased the b-dimension by forcing the oxygen triads open. With small cations the b-dimension decreased as a function of the residual water content rather than of cation size. The OH stretching vibration did not change in position, probably due to the inclined orientation of OH groups. The cations perturbed the lattice vibrations when they caused a change in the tetrahedral rotation angle, i.e. in b-dimension. With the small cations Be, Li and Mg, librations below 450 cm−1 shifted towards higher frequencies indicating cation migration into the empty octahedral sites.