X-ray diffraction and high-resolution transmission electron microscopy showed that cesium ions (Cs+'s) in a vermiculite clay formed a segregated monoionic layer in the interlayer spaces. Each ion was fixed simultaneously at the centers of hexagonal rings in the upper and lower silicate tetrahedral sheets. No Cs+ was eluted even when Cs+-vermiculite was kept at 80 degrees C for 48 h in the presence of a large excess of Al(NO3)(3) or [Co(NH3)(6)]Cl-3.