For improving chemical and thermal stability of -Al₂O₃ membranes boehmite (AlOOH) sol-particles are coated with Zr⁴⁺-species with two techniques. These techniques are heterogeneous precipitation (HP-method) and surface-reaction-followed-by-polycondensation (SRPC-method). A continuous coating layer is formed at relative low Zr⁴⁺-concentration, about one monolayer coverage of boehmite particles, and with the HP-method. For larger concentrations and for the SRPC-method small particles (order 1 nm) of Zr⁴⁺-species are formed on the surface of the boehmite particles. After drying and calcination up to 1000°C no continuous layer of a zirconia phase could be detected for all samples. However the thermal stability of the porous structure is improved. Phase transitions of alumina occur at temperatures of 1100°C and the porous structure of the membrane material is then destabilized.