A microporous membrane has been prepared by coating alumina support with evaporated polydimethylsilane, followed by curing and pyrolysis. Permeation properties of the membrane exhibit gas separation capability. At permeation temperature 180degreesC, helium permeance is 1.1 x 10(-9). nitrogen 1.5 x 10(-10). and iso-butane 3.7 x 10(-11) mol s(-1) Pa-1 m(-2). The membrane permeance is enhanced when the pyrolysis is carried out in the presence of low-pressure steam, and its percentage increase rises with the pyrolysis temperature. But the membrane selectivities of permanent gas pairs are damaged. (C) 2004 Elsevier B.V. All rights reserved.