After radionuclides pollution of soils by nuclear power plant accidents, Cs + strongly interacts with swelling clay minerals, which in turn control the fixation or the migration of this pollutant in the natural media. Little is known, however, about the role played by the amount of layer charge and the two-dimensional confinement on Cs + fixation. In this study a series of tetrahedrally charged smectites with structural formula inter [M + x ] oct [Mg 6 ] tet [Si 8−x Al x ]O 20 (OH) 4 with a layer charge (x) varying from 0.8 to 2.0 and with Na + or Cs + as interlayer cation M + were used. While Na + remains fully exchangeable over the entire range of layer charges investigated, the fraction of exchangeable Cs + decreased gradually for samples with x ≥1.4/O 20 (OH) 4. Structure analysis of Cs +-samples in water-saturated or at 80% relative humidity conditions showed a gradual decrease of the layer-to-layer distance. The results were found to qualitatively agree with analysis of the thermodynamics of the clay/water interfaces derived from molecular simulations. Quantitative structure analysis by modeling of 00ℓ reflections on experimental Xray diffraction patterns suggested that exchangeable Cs + can be correlated with the amount of hydrated interlayers, whereas fixed Cs + in these conditions corresponds to the amount of collapsed layers. To describe the coexistence of both exchangeable and fixed Cs + , a toy-model accounting for the presence of heterogeneous charge distribution was used, in agreement with experimental data obtained on these samples. This model successfully interprets the overall reactivity of Cs + towards smectite over a large range of layer charge values and may contribute to an improved description of Cs + mobility in contaminated soil environments. Water density profile Z-position (Å) Sodium density profile Water density profile Z-position (Å) Cesium density profile Water density profile Z-position (Å) Sodium density profile Water density profile Z-position (Å) Cesium density profile a. Na-Sap-0.8 c. Na-Sap-1.8 b. Cs-Sap-0.8 d. Cs-Sap-1.8