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High-temperature stability of Pd-based membranes benefits their application in steam reformers and sulfur-contaminated H2 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 °C and 650 °C. Remarkably, PdCu membranes were much more stable than Au-containing ones. The H2 permeation rates of some PdAu and PdCuAu membranes declined at 550 °C with substantially increasing N2 fluxes. This was triggered by severe morphological deformation of the Au alloy films into stoichiometrically inhomogeneous, cavernous structures. The H2 fluxes of the PdCu membranes started to decline at 650 °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 °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 °C during operation in H2. The instability of PdAu membranes at high temperatures may be mitigated by addition of sufficient Cu to obtain ternary membranes with good H2 permeability and better thermal stability.