The outer surface of an alpha-alumina tube was coated with a gamma-alumina film formed by a sol-gel process. Then the tube was coated with polycarbosilane (PC), which was cured at 473 K and pyrolyzed at 623-823 K. The composite membrane prepared was ca. 1-1.4 mu m in thickness and had no pinholes larger than several nm. For the composite membrane pyrolyzed at 623 K, the permeation of H-2 and N-2 at 283-673 K was controlled by the activated diffusion mechanism, and for the membrane pyrolyzed at 823 K by the Knudsen diffusion mechanism. The permeation of CO2 was facilitated by the surface diffusion mechanism for all membranes prepared. The optimum pyrolysis temperature was 723 K from the viewpoints of the permeance of H-2 and the separation factor of H-2 to N-2, which were respectively 5.5 X 10(-7) mol m(-2) s(-1) Pa-1 and 7.2 at 673 K.