Separation of ions from aqueous solutions was performed by reverse osmosis (RO) on an a-alumina-supported MFI-type zeolite membrane synthesized by in-situ crystallization. For the 0.1 M chloride single-salt solutions, the separation efficiency in terms of ion rejection was found to increase with the ion valence in the order r(Al)(3+) > r(Mg)(2+) > r(Na)(+), while the ion and water fluxes changed in the reverse order. The charge density, size, and apparent dynamic hydration number of the ion as well as the mobility of the hydrated ion were found to have critical influences on ion diffusion and water permeation through the polycrystalline zeolite membrane.