Highly (h0h)-oriented silicalite-1 membranes were reproducibly prepared on the inner surface of a-alumina tubes by secondary (seeded) growth of a randomly oriented seed layer. The ideal selectivities of H-2/SF6 and N-2/SF6 were 2400 and 1500, respectively, indicating that the membrane had few boundary defects. The best silicalite-1 membrane displayed n-butane permeance of 3.1 x 10(-7) mol (m(2) s Pa)(-1) (= 926 GPU) and n-butane/i-butane separation factor of 29 for an equimolar n-butane/i-butane binary mixture at 333 K. Preferentially oriented and uniform silicalite-1 layers are responsible for the high separation performance. Mixture n-butane/i-butane separation factors were always higher than ideal selectivities. The effects of temperature, pressure, feed concentration, feed flow rate, sweep gas flow rate and water vapor in feed on the membrane performance were studied. The separation mechanisms including preferential adsorption, molecular sieve and competitive diffusion were discussed for the n-butane/i-butane separation through the silicalite-1 membranes.