ZrO2–Al2O3 composite membranes with 50 mol% zirconia were prepared by sol–gel process on the porous ceramic tube. X-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR), nitrogen isothermal absorption measurement and gas permeability test were used to characterize the membranes. It was identified by XRD that, from 700 to 1100 °C, the composite membranes existed in only tetragonal ZrO2, and neither monoclinic ZrO2 nor alpha Al2O3 appeared. The thermal stability of the composite membranes was improved greatly, compared to that of pure Al2O3 or ZrO2 membranes. Scanning electron microscopy (SEM) images showed that the membranes were defect-free. The most concentrative pore diameter of the composite membranes determined was about 4.3 nm. Gas permeability test suggest that the composite membranes possessed gas selectivity, and the separation factors for N2/Ar were 1.119 at 0.3 MPa and 1.140 at 0.5 MPa, respectively, which were slightly smaller than the value expected from the ideal Kundsen diffusion (α=1.194).