Abstract: Molecular dynamics (MD) is used to investigate the structural and diffusive properties of interlayer species, namely water and counterion speciations of Na-montmorillonite and Na/Mg-montmorillonite. It is found that Na/Mg-montmorillonite exhibits a higher swellability compored with that of Na-montmorillonite for given water content, especially with 48～72 water molecules of interlayer, the expansion of layer spacings of Na/Mg-montmorillonite is greater than those of Na-montmmorillonite. Interlayer water molecules have a stronger tendency to solvate Mg2+ compared to that of Na+. Inner-sphere complexes and outer-sphere complexes of Mg2+ with water are observed, whereas only planar inner-sphere complexes of Na+ with water are found for two-layer Na/Mg-montmorillonite hydrates. The results also indicate the different diffusion modes of Na+ and Mg2+ for Na/Mg montmorillonite: Na+ remains close to the surfaces of the sheets, whereas Mg2+ is easily hydrated and is located in the middle of the interlayer. Comparatively, Na+ has a wider diffusion range and higher self-diffusion coefficient than Mg2+. Moreover, self-diffusion coefficient of interlayer water is higher for Na-montmorillonite than that for Na/Mg-montmorillonite at the same water content. Most of the results obtained in this study can be understood in terms of the strong solvation of Mg2+ with water molecules, and Mg2+ and Na+ have different influences on the structure of interlayer. It can therefore be assumed that the replacement of a relatively small amount of interlayer Na+ cations by Mg2+ in Na-montmorillonite during the initial stage can substantially change the swelling and diffusion properties of interlayer species.