A porous alpha-alumina tube of 2.4 mm o.d. with an average pore size of ca. 150 nm was coated with p-xylene solutions of polycarbosilane (PC) and polystyrene (PS), The weight fraction of PS to total solid mass (PC + PS) was varied as 0, 1, 3 and 5 wt% (denoted as PC-PSx for a solution of x wt%). Then the film was cured in air at 473 K and pyrolyzed in argon at 1223 K. The procedure of dipping, drying, and pyrolysis was repeated three or four times in total. An Si-C-O membrane that was prepared with the PC-PS1 solution exhibited an H-2 permeance of 4 x 10(-8) mol.m(-2).s(-1).Pa-1 and an H-2/N-2 selectivity of 20 at 773 K. The PS was concentrated into ellipsoidal domains due to phase separation during the curing step, and it was depolymerized and dissipated during the pyrolysis step. All membranes were resistant to heat treatment at 1223 K in argon for 20 hours. Except for the PC-PSS membrane, voids that were formed by the decomposition of PS did not perforate the top layer, When the PC-PSI membrane was exposed to a mixture of steam and helium at 773 K, the permeance was decreased to half in the first day, but was not greatly changed during the next 2 days.