Microporous ceramic membranes of Al2O3, BaO-Al2O3 and La2O3-Al2O3 were prepared by repeated dip-coating of a sol on an alpha-Al2O3 porous substrate. The thermal stability of the membranes was studied from their pore structure, gas permeability, gas separation factor and surface microstructure after calcination at 500-1300 degrees C. The BaO-Al2O3 and La2O3-Al2O3 membranes exhibited much higher heat resistance than the Al2O3 membrane. Although significant crack formation and pore growth were observed for the Al2O3 membrane after heating above 900 degrees C due to gamma-alpha transformation, the BaO-Al2O3 and La2O3-Al2O3 membranes retained a crack-free microstructure, narrow pore size distribution, and excellent selectivity for gas permeation even after calcination up to 1100 degrees C.