Yttria stabilized zirconia (YSZ) polymeric sols were synthesized by controlled hydrolysis and condensation of zirconium tetra-n-propoxide. Yttrium nitrate was added before hydrolysis as the source of yttria. Acetylacetone was used as chelating ligand to modify the reaction rate of zirconium alkoxide. The dependence of YSZ sol formation as well as gelation time was investigated experimentally on the contents of acetylacetone, n-propanol (solvent) and water for hydrolysis. With a stable sol, YSZ membranes were prepared on porous a-alumina supports by a dip-coating process. The membranes were characterized by scanning electron microscopy and gas permeability testing. Unsupported YSZ membranes were also prepared with the same sol, and were investigated with isothermal nitrogen adsorption/desorption porosimetry and X-ray diffraction. The results show that YSZ membranes with nanosize pores were successfully prepared by the sol