MFI-type zeolite membranes with different Si/Al ratios were synthesized by seeding and secondary growth. Several factors including alkalinity, synthesis time, and Si/Al ratio were varied to optimize the synthesis conditions. Ion separation of 0.10 M NaCl solution by zeolite membranes was investigated through a cross-flow reverse osmosis system. The aluminum content was found to play an important role in zeolite growth and reverse osmosis separation performance afterward. Membrane hydrophilicity and surface charge measured by the contact angle and particle zeta-potential were found to be strongly dependent on the aluminum composition of the membranes. Both water flux and ion rejection increased considerably as aluminum ions were incorporated into the zeolite framework with a water flux increase from 0.112 to 1.129 kg/m(2)center dot h and ion rejection improvement from 90.6% to 92.9%. The enhancement in ion separation and water flux for high aluminum ZSM-5 membranes was further analyzed by enhanced water affinity on the membrane surface and restricted ion permeation in confined spaces.