Palladium-based membranes are attractive for their nearly perfect permselectivity to hydrogen. Membrane modules, consisting of a membrane foil, porous stainless steel substrate, test frame and flange were assembled and tested in an electrically heated vessel. Instantaneous hydrogen permeation flux was measured. Influences of operation conditions on the membrane performance were examined. Microstructure and morphology of the membrane surface and the cross-sectional surface of the substrate and membrane foil were characterized by scanning electron microscopy. It was observed that for an operation temperature higher than 755 K, the hydrogen permeation flux through the membrane module with 0.2 mu m grade porous 316L stainless steel substrate decayed continuously due to the inter-metallic diffusion between the membrane and the substrate. For a temperature of around 869 K-943 K, a stable hydrogen permeation flux through the membrane module with 0.5 mu m grade stainless steel substrate was observed. Pretreatment of the 0.5 mu m grade substrate with polishing and etching helped to smooth the membrane foil surface. However, it changed the surface structure of the material and led to a decrease in hydrogen permeability. Under the conditions investigated, the permeation factor of the module increased by raising the hydrogen pressure in the vessel side and decreasing the membrane module temperature. By decreasing the hydrogen exit partial pressure by sweep gas, the membrane module permeation flux increased, while the permeation factor decreased. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.