Composite metal membranes consisting of a palladium coating layer applied over a vanadium base-metal layer exhibit rapid decline in hydrogen flux when operated at 700 degrees C, Cross-sectional analyses of these membranes show that intermetallic diffusion between the palladium and vanadium layers also occurs rapidly under these conditions. The diffusion of vanadium into palladium is found to be faster than the diffusion of palladium into vanadium. Significantly, we find that hydrogen increases the rate of intermetallic diffusion, and that flux decline correlates to the degree of intermetallic diffusion. An explanation of these events is put forward. Finally, we find that a layer of porous aluminum oxide placed between the palladium layer and the vanadium imparts stability to the membrane-i.e. neither flux decline nor intermetallic diffusion are observed at 700 degrees C.