The permeance of Pd-Cu alloys containing 40, 53, 60, and 80 wt.% Pd has been determined over the 623-1173 K temperature range for H-2 partial pressure differences as great as 2.6 MPa. Pure palladium and copper membranes were also evaluated. The Pd-Cu alloys exhibited predictable permeances that reflected the crystalline phase structures as shown in the binary phase diagram. Under conditions of face-centered-cubic (fcc) stability, the permeance increased steadily with palladium content, approaching the permeance of pure palladium membranes. The 53 and 60 wt.% Pd alloys were evaluated at temperatures within the body-centered-cubic (bcc) stability region. For both alloys, the bcc permeance was several times greater than the fcc permeance with the 60 wt.% Pd bcc permeance at 623 K reaching about 70% of the permeance of palladium. These bcc alloys were subjected to temperature increases during testing that resulted in transition from bcc to fcc, followed by temperature decreases that should revert the alloys to bcc. The permeances dropped abruptly during the transition from bcc to fcc. However, on cooling back to the bcc stability region, neither the 60 nor 53 wt.% Pd alloy completely regained a bcc permeance during the test period. All of the Pd-Cu alloys subjected to testing at H 73 K showed some permeance decline that was attributed to intermetallic diffusion between the membrane and support. The application of a diffusion barrier between the support and membrane foil in a 53 wt.% Pd permeance test successfully blocked the intermetallic diffusion and prevented degredation of the membrane's performance. (C) 2004 Elsevier B.V. All rights reserved.