Yttria stabilized zitconia (YSZ) powders comprised of Y/Zr in molar ratios of 0.01, 0.04, and 0.08, were prepared by a sol-gel method and their crystal structures and pore size distributions were determined. The 8-mol% Y-doped zirconia (8YSZ) powder restrained the crystallization and had the smallest pores. Porous 8YSZ membranes, prepared on a porous a-alumina support tube by a sol-gel method, showed a low permselectivity of 5.1 for H-2 to CO2. The membranes were modified by impregnation with Pt or Ni to improve the H-2 permselectivity. The perm-selectivities of H-2 to CO2 at 500degreesC through the Pt-YSZ and Ni-YSZ membranes were increased to 22 and 29, respectively.