High-temperature stability of Pd-based membranes benefits their application in steam reformers and sulfur-contaminated H-2 streams because both membrane reforming efficiency and sulfur tolerance of Pd alloys increase much with temperature. Hence, we investigated PdCu, PdAu, and PdCuAu membranes supported on porous ceramic tubes between 500 degrees C and 650 degrees C. Remarkably, PdCu membranes were much more stable than Au-containing ones. The H-2 permeation rates of some PdAu and PdCuAu membranes declined at 550 degrees C with substantially increasing N-2 fluxes. This was triggered by severe morphological deformation of the Au alloy films into stoichiometrically inhomogeneous, cavernous structures. The H-2 fluxes of the PdCu membranes started to decline at 650 degrees C with leak flows increasing slightly. Moreover, the PdCu layer morphology remained dense and compositionally homogeneous even after testing for up to 4800 h between 500 and 650 degrees C. The strikingly different high-temperature stability can be understood by considering the divergent surface segregation tendencies of Cu and Au and their differing impact on hydrogen solubility in Pd alloys. As a result, Au may desorb much more easily from membranes than Cu leading to structural instability above 500 degrees C during operation in H-2. The instability of PdAu membranes at high temperatures may be mitigated by addition of sufficient Cu to obtain ternary membranes with good H-2 permeability and better thermal stability.