Thin (7 mum) polycrystalline zeolite membrane with high permeances was reproducibly prepared on an alpha -Al2O3 support, whose pore size is about 2 mum, by a two-stage varying-temperature synthesis. Compared to the synthesis at constant temperature, equiaxed particles in the membrane were achieved; thin and dense membrane was obtained. Since nucleation occurred during an induction period and large amounts of nuclei could form at low temperature, for ZSM-5 membrane preparation, if the temperature change were made after completion of the nucleation period, nuclei on the support would grow quickly to form thin and continuous membrane. The membrane was characterized by XRD, SEM and permeation measurements using single gases and mixtures of n-butane and iso-butane. At room temperature, the highest H, permeance is 2.4 x 10(-6) mol/m(2) s Pa, the highest ideal selectivities are 129 for H-2/n-butane, 1548 for H-2/iso-butane. The separation selectivity for n-butane/iso-butane has maximum (e.g. 78 at 373 K) as a function of temperature and is higher than the ideal selectivity, It is probably that n-butane effectively blocks the permeation of iso-butane in a mixture, which decreases the permeance of iso-butane.