" This work investigated the long-term stability of sulfur tolerant Pd/Cu alloy membranes for hydrogen separation by performing characterizations lasting several thousand hours in H2, He and H2S/H2 atmospheres ranging in concentration from 0.2 – 50 ppm and temperatures ranging from 250 - 500ºC. Two methods were used for fabricating the Pd/Cu membranes so that the sulfur tolerant fcc alloy would remain on the surface and minimize the decrease in hydrogen permeance inherent with fcc Pd/Cu alloys. The first method consisted of annealing a Pd/Cu bi-layer at high-temperatures and the second consisted of depositing a Pd/Cu/Pd tri-layer with an ultra-thin surface alloy. High temperature X-ray diffraction (HT-XRD) was employed to study the kinetics of the annealing process and atomic adsorption spectroscopy (AAS) was used to investigate the kinetics of the Cu deposition and Pd displacement of Cu. Upon the introduction of H2S, the permeance decrease observed was dependent upon the H2S feed concentration, and not the time of poisoning. However, after the recovery in pure H2 there was a portion of the permeance which could not be recovered due to adsorbed sulfur blocking H2 adsorption sites. The amount of recoverable permeance was dependent on the time of exposure to H2S and reached a limiting value which decreased with temperature. X-ray photoemission spectroscopy (XPS) was used to investigate poisoned samples and it was observed that the permeance not recovered at a given temperature in H2 was caused mostly by Cu sulfides. Both bi-layer and tri-layer membranes had hydrogen permeances which were higher than homogeneous Pd/Cu membranes of the same surface concentration. However, the tri-layer membranes performed as well as Pd membranes thus eliminating the disadvantage of alloying Pd with Cu without sacrificing sulfur tolerance. "