ZSM-5 zeolite membranes with reproducible properties were prepared by in-situ synthesis on porous alpha- and gamma-alumina tubular supports and characterized by XRD, SEM and electron microprobe analysis. Single-gas permeances for H-2, CH4, N-2, CO2, n-butane, and i-butane increase over some temperature range, but some gases exhibit maxima or minima. The highest ideal selectivities at room temperature are 299 for N-2/SF6, 392 for H-2/n-butane, and 2,820 for H-2/i-butane. These membranes can separate n-butane/i-butane, H-2/n-butane and H-2/i-butane mixtures. All n-butane/i-butane separation selectivities have maxima as a function of temperature and are higher than ideal selectivities because n-butane inhibits i-butane permeation. Thus, separation is not by size selectivity but is due to pore blocking. Temperature dependencies of single-gas permeances and separation selectivities depend strongly on the location of zeolite crystals and the location is determined by preparation procedure. Ideal selectivities also depend strongly on the preparation procedure. When the zeolite forms a continuous layer on the inside surface of the support tubes, pure i-butane permeates faster than pure n-butane so that the single-gas permeances are not determined just by molecular size. The i-butane permeance also increases much more with temperature than the n-butane permeance. The permeation behavior may be the result of permeation through nonzeolitic pores in parallel with zeolite pores. When zeolite crystals are dispersed throughout the pores of alpha-alumina supports, permeances are lower and gas permeation and separation properties are quite different Ideal selectivities are lower, pure n-butane permeates faster than i-butane, and the permeances increase much less with temperature. Separation selectivities are lower but can be maintained to higher temperatures.