Silicalite membranes were prepared by in situ crystallization on the Outer surface of porous tubular mullite supports with tetraethoxysilane (TEOS) and tetrapropylammonium hydroxide (TPAOH) as silica source and organic structure directing agent, respectively. The outer-surface of the porous support was completely covered with randomly oriented, intergrown silicalite crystals and the thickness of the dense intermediate layer was estimated about 5-10 mu m, judging from the scanning electron microscopy observation. Pervaporation of organic/water mixtures through silicalite membranes was investigated. The silicalite membranes showed high organic selectivity from organic/water mixtures. For example, the highest acetone/water and methyl ethyl ketone (MEK) /water separation factors of 934 and 32,000 with fluxes of 0.20 and 0.25 kg m(-2) h for a feed of 1 wt.% acetone and 5 wt.% MEK at 30 degrees C, respectively, were obtained for the silicalite membrane prepared at 185 degrees C for 40 h of hydrothermal treatment. An n-butane/i-butane ideal separation factor of 73 at 200 degrees C and at pressure difference of 101 kPa also indicated that the silicalite membrane prepared under such synthesis conditions had fewer defects.