The thermal stability of palladium (Pd)-based alloy membranes is the integral part for their practical applications in hydrogen recovery from mixed gas. While many research groups have developed porous ceramic supported Pd-based alloy membranes, their long-term thermal stability at elevated temperatures are not almost investigated so far. We examined the change in hydrogen permeability and gas selectivity with time of thin palladium-silver (Pd-Ag) alloy membranes (less than 5-μm thick) supported on porous α-Al2O3 substrates at temperatures of 300-850°C. When the composite membranes were exposed to hydrogen below 550°C, they showed excellent permselectivity for hydrogen during long-term gas permeation tests. The hydrogen permeation flux across the Pd membranes significantly enhanced by Ag alloying, reaching 1.85molm-2s-1 at 550°C in an alloy membrane containing 20wt.% Ag. However, an appreciable decrease in the hydrogen permeation flux was observed above 600°C. SEM-EDX and XPS analyses indicated that a considerable amount of aluminum (Al) penetrated the Pd-Ag alloy membrane layer. Highly active atomic hydrogen present at the interface between the alloy membrane and the porous α-Al2O3 substrate induced the reduction of Al2O3 to Al and caused significant migration of Al atoms into the alloy layer. Consequently, the Pd-Ag/α-Al2O3 composite membranes lost their hydrogen permeability during operations at 600°C. © 2010 Elsevier B.V.