Morphological change of Pd and Pd-25Ag membranes supported by V-15Ni alloy upon hydrogen permeation was investigated in the temperature range 423-673 K. The supported Pd-25Ag membrane exhibited higher resistance to hydrogen-induced cracking and grain growth than the supported Pd membrane. Long-term permeation of Pd-25Ag/V-15Ni composite membrane was carried out at 573 and 673 K for 200 h. There was no strong metallic interdiffusion between the Pd-25Ag membrane and the V-15-Ni support after the long-term permeation at 573 K but small amounts of oxide had formed on the surface of Pd-25Ag membrane. Whisker and fissure-oxide morphologies were dominant on the exit and entrance side of the Pd-25Ag/V-15Ni composite membrane, respectively, accompanied by severe metallic interdiffusion after the long-term permeation at 673 K. AES and FE-SEM results revealed that metallic interdiffusion and selective oxidation of vanadium were responsible for the deterioration of Pd-25Ag membrane at 673 K. Hydrogenation-dehydrogenation of Pd and Pd-25Ag membranes supported by stainless steel and V-15Ni alloy were in situ examined by an optical microscope. The formation of hydride was uniform in the Pd/V-15Ni sample but localized in the Pd-25Ag/V-15Ni sample, suggesting that the hydrogen transfer through interface was strongly dependent on the composition of Pd alloy membranes. As for the stainless steel supported samples, both Pd and Pd-25Ag membranes had fractured. (C) 2004 Elsevier B.V. All rights reserved.