Swelling clays present a multi-scale structure and therefore a multi-scale porosity hierarchy macroporosity between aggregates / inter-particle mesoporosity / microporosity in the interlayer spacing, as described by mercury intrusion porosimetry, nitrogen adsorption and XRD experiments respectively. In this work, XRD measurements, water adsorption and calorimetry, thermoporometry, and electrical conductivity results are coupled with electrostatic calculations to investigate (i) the hydration sequence occurring in alkali/alkali-earth cations exchanged montmorillonites, and (ii) the diffusion properties of the interlayer cations. The comparison between montmorillonites saturated with homoionic alkali or alkali-earth cations and montmorillonite saturated with both alkali and alkali-earth cations leads us to elucidate the impact of the nature of cations and the possible interactions existing upon hydration.Furthermore the determination of the driving forces for hydration in swelling clays as a function of the nature of the interlayer cations allows us to better capture the dynamics of ionic exchange in swelling clays.