This paper reports the preparation, characterization and stability tests of thin-film Pd–Ag supported membranes for high-temperature fluidized bed membrane reactor applications. Various thin-film supported membranes have been prepared by simultaneous Pd–Ag electroless plating and have been initially sealed with a sealing procedure previously validated for Water gas shift (WGS) application (400 °C). The membranes have been characterized for single gas and mixed gas permeation, and for methane steam and autothermal reforming in a fluidized bed membrane reactor at 550–600 °C using a Ru-based catalyst. In addition, the performance of these membranes was compared to commercial membranes from REB Research & Consulting under the same reaction conditions. The applied sealing showed nitrogen leaks at 600 °C and different sealing approaches were tested solving this problem. Finally, also the long-term stability of the thin-film Pd–Ag supported membrane at 600 °C has been investigated. Keywords: Palladium membrane preparation; Fluidized bed membrane reactor; Membrane stability; High temperature sealing; Autothermal steam reforming