Pd-Cu alloys are being considered for hydrogen membrane applications because of their resistance to sulfur poisoning. Therefore the permeance of Pd-Cu alloys containing 53, 60, and 80 wt% Pd has been determined over the 623 – 1173 K temperature range for H2 partial pressure drops as great as 2.75 MPa. The results indicate that Pd-Cu alloy composition and thermal history influence membrane permeance. The 60%Pd-40%Cu alloy exhibited very high permeance at 623 K, although both the 53%Pd and 60% Pd alloys exhibited a distinct drop in permeability at higher temperatures due to the transition of the Pd-Cu crystal structure from bcc to fcc. Upon cooling the membrane back to 623 K, the permeability of the 60%Pd alloy was initially an order-of-magnitude less than its initial value, but the permeance increased steadily with time as the Pd – Cu crystal structure slowly reverted to bcc. The fcc 80%Pd alloy was less permeable than the bcc 60% Pd alloy at 623 K, but the 80% Pd alloy was more permeable than the fcc 60%Pd alloy at elevated temperatures.