Zeolites are known for their uniform pore size and are therefore suitable as membrane materials for the separation of different kind of molecules. The selectivity of these zeolite membranes is determined by the size of these molecules and by the diffusion and adsorption processes. These processes are investigated by molecular. dynamic simulations. The simulations can give, at an atomic level, detailed insights in the diffusion through the membrane and in the adsorption of the molecules in the membrane. Here we want to investigate the influence of the exchange of calcium in an NaY membrane on the diffusion of methanol through a faujasite-type membrane.