Mesoporous yttria stabilized zirconia (YSZ) membranes can be used as supports for ultrathin dense ceramic or metallic membranes, and for ultrafiltration (UF) applications in harsh environments. This paper reports synthesis and characterization of sol-gel derived UF YSZ membranes. 0.25 M zirconia sol was prepared by hydrolysis and condensation of zirconium n-propoxide. A solution-sol mixing method was used to dope 8 mol% yttria in zirconia. Supported YSZ membranes were prepared by dip-coating the yttrium doped zirconia sol on the porous a-alumina substrate, followed by drying and calcining under controlled conditions. The membranes prepared in this study are of cubic fluorite phase. Helium permeation experiments show that the supported membranes are pin-hole (or crack) free. The 5 time dip-coated membrane determined by SEM micrograph is about 3.5 mu m in thickness, with an average pore diameter of 3 nm.