Molecular sieve zeolite membranes have recently been shown to be potentially useful for treating saline wastewater by reverse osmosis. In this paper, reverse osmosis of 0.1 M solutions of alkali metal chlorides, including LiCl, NaCl, KCl, RbCl, and CsCl, has been investigated on MFI-type zeolite membranes synthesized from an aluminum-free precursor. The MFI zeolite membrane exhibited high rejection rates (r(i) > 95%) for all alkali metal ions at an applied pressure of 2.75 MPa. The rejection rates of Li+ and Na+ declined slightly while the ion flux increased more dramatically than did the water flux with moderately raising the temperature. Increasing the hydraulic pressure difference across the membrane resulted in significant enhancements in both the water flux and the ion rejection rates but with much less influence on the ion flux. The ion flux increased monotonically with the crystallographic ion size from Li+ to Cs+ while the dependence of the corresponding water flux on ion size exhibited a maximum for the 0.1 M KCI solution. The ion separation by reverse osmosis on the MFI zeolite membranes was found to rely on the restricted access of hydrated ions to the zeolitic pores and the competitive diffusion of water and ions in the zeolite channels. (c) 2006 Elsevier B.V. All rights reserved.