We studied the interaction of cesium cation in the aqueous phase with soil clay by combining microscopic and macroscopic data. Investigations concerning selective sorption of cesium by this soil clay are presented. The sample studied is selective for concentrations <2×10−3 mol/l in solution. Far-infrared (FIR) shows the presence of two selective adsorption sites. At higher loadings, 133Cs MAS-NMR shows that most cesium is essentially adsorbed on external sites which are not very selective. FIR and adsorption data lead to the conclusion that collapsed illite clay is responsible for the selectivity observed at low concentration. However, cesium cations can diffuse between illite layers only if these layers are involved in a mixed smectite–illite mineral.