Tubular oriented silicalite-1 membranes were prepared on the inner surface of a-alumina tubes by secondary (seeded) growth. Membrane microstructures including orientation, thickness and defect densities which affected membrane performance could be regulated by synthesis optimization for butane isomer separation. Synthesis parameters such as synthesis temperature, synthesis time and solution composition were modified. Membrane orientation was greatly affected by synthesis temperature. Two typical membranes of highly (h0h)-oriented and mixed (h0h)&c-oriented membranes were obtained after synthesis optimization. Four highly (h0h)-oriented silicalite-1 membranes prepared under optimized synthesis conditions showed ideal H-2/SF6 selectivities of 1325 +/- 35, which was much higher than that of mixed (h0h)&c-oriented membrane M3 (300), indicating that the former membranes had fewer boundary defects. These highly (h0h)-oriented membranes displayed n-butane permeances and separation factors were (2.26 +/- 0.23) x 10(-7) mol (m(2) s Pa)(-1) and 32.7 +/- 2.5 for an equimolar n-butane/i-butane mixture at 333 K, respectively. The low deviations in permeance and selectivity indicate that membrane synthesis had a good reproducibility. This average n-butane permeance for these highly (h0h)-oriented membranes was 4 times higher than that of mixed (h0h)&c-oriented membrane M3. Both kinds of highly (h0h)- and (h0h)&c- silicalite-1 membranes displayed the similar trend with test temperature and pressure.