The effects of synthesis parameters (i.e. [TEOS], [TPA(+)], [OH-], dilution and temperature) on the membrane thickness and orientation were investigated for MFI zeolite membranes grown onto seeded supports (i.e. porous glass and alumina). The zeolite crystals grew preferentially with its c-axis aligned normal to the support surface. This gave the membrane a preferred orientation of (101) at temperatures below 433 K and an orientation of (002) at higher temperatures. An empirical model for zeolite membrane growth as a function of synthesis variables was derived based on the experimental data. The model gave an adequate description of the membrane growth and provided an excellent guide for the synthesis of zeolite membranes with engineered microstructure. This model was employed in preparing zeolite membranes of different thickness (tau = 2-8 mum). Helium, argon and sulfur hexaflouride were used as molecular probes for investigating the transport pathways through the membrane microstructure.