Molecular dynamics simulations were used to investigate the underlying mechanisms of water interaction with Wyoming-type montmorillonites. The study focused on (1) the clay swelling process for different cations with the same charge (Na+ and K+), and (2) the stability of clay structures under insertion of water. Simulations show that the ionic radius of the cations and their hydration energies seem to be the critical factors that determine the distribution of cations and water molecules in the interlayer region. The differences in cation distribution are the microscopical explanation as to why saturation of smectites with K+ cations retards clay damage, when compared to saturation with Na+ cations.