Desorption of Cs+ fixed in collapsed interlayer region of vermiculite was studied. Monovalent cations readily induced interlayer collapse inhibiting Cs+ desorption. Larger hydrous ionic radii of divalent cations greatly prevented Cs+ desorption. Effect of divalent cation on Cs+ desorption changes depending on thermal treatment. ∼100% removal of saturated Cs+ was achieved by hydrothermal treatment at 250 °C.