The suitability of swelling clays as a barrier to isolate nuclear waste in deep geological disposal is based on their microstructure, characterized by pore sizes down to the nanometre scale, and by their physico-chemical properties such as an excellent retention capacity for many radionuclides. A process-based understanding of the key features of radionuclide migration in such environments is required to demonstrate a valuable performance under all feasible conditions. In this study, the diffusion of tritiated water (HTO) and ²²Na through Na-montmorillonite was studied as a function of the bulk dry density, the concentration of the background electrolyte (0.1, 1 and 5 M NaClO₄) and the temperature (between 0 and 80 °C). We observed that the diffusion of neutral species, such as water, depended on temperature and the bulk dry density only, while the diffusive fluxes of ²²Na⁺ were additionally influenced by the concentration of the background electrolyte. The experimental data were in agreement with both a single porosity and a parallel flux model demonstrating the validity of both approaches for rather low bulk dry densities and high concentrations of the background electrolyte. Some discrepancies between the model predictions and the experimental data were, however, noted. Assuming a reduced molecular mobility in the clay phase strongly reduced those discrepancies. Activation energies measured on samples with high bulk dry density were slightly higher than values measured for bulk water (17 kJ mol⁻¹), those on low bulk dry density samples rather smaller. The activation energies of ²²Na⁺ and HTO were similar indicating possibly a dynamical coupling of the diffusion of water and cations.