A thin wall Pd-Ag tube (thickness 50 mum) obtained by a procedure of cold-rolling and annealing of thin metal foils has been characterized in long-term tests for determining the hydrogen permeability under operating condition of a temperature range from 300degreesC to 400degreesC and transmembrane differential pressure from 50 to 100 kPa. During testing, the physical and mechanical stability of the rolled membranes have been observed and high hydrogen fluxes have been measured. The long-term tests have also shown the modification of the surface structure of the membrane due to the hydrogen-metal interaction and the thermal cycling: as a consequence, an increase of the mass transfer properties of the hydrogen through the material (diffusivity and solubility) has been observed and the membrane tube has attained very high performances in about 3 months of operation in terms of hydrogen permeability. Furthermore, the tests have demonstrated that these Pd-Ag membranes have the capability to separate hydrogen from gas mixtures with a complete hydrogen selectivity and can be used to produce ultra-pure hydrogen for applications in energetic fields in membrane reactors by molecular reforming in membrane reactors (i.e. isotopic hydrogen separation, fuel cell, etc.). (C) 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.