Molecular simulations using the Cerius2 modelling environment have been used to investigate the structure of montmorillonite (MMT) intercalated with Zn cations. Basal spacing and bonding of Zn2+ cations in the interlayer have been investigated with regard to their dependence on the water content. In the first part of the work, Zn cations in the interlayer are coordined by six water molecules and the energy of the system is discussed. The energy discussion is focused on the influence of the different starting orientations of the Zn octahedron, and different positions of Mg atoms in the octahedral sheet. An energy scan at fixed d-spacing was carried out. Later the hydration simulation giving the hydration curve (i.e. the dependence of d-spacing on the number of water molecules in the interlayer) was also carried out. Two different hydration states were found — for certain humidity ranges there is almost constant d-spacing and between these intervals there is a sharp edge. Finally the structure of totally dehydrated Zn-MMT was simulated.