The high-temperature, water-gas shift reaction was conducted in 100wt%Pd and SO wt%Pd-20wt%Cu (Pd8owt%Cu) shell-and-tube membrane reactors at 1173 K with a 241 kPa (35 psig) trans-membrane pressure differential in the absence of heterogeneous catalyst particles. The tube bundle consisted of four parallel 15.25 cm long, 3.175 mm CD Pd-based tubes with a wall thickness of 125 mu m. The modest catalytic activity of the Pd-based membrane surface for the forward WGSR, the high rate of hydrogen extraction through the Pd-based membranes, and the long residence times (1-5 s) resulted in a dramatic shift in carbon monoxide conversions of 93% at 1173 K and a 1.5:1 steam-to-carbon monoxide feed ratio-a value well above the equilibrium value of similar to 54% associated with a conventional (non-membrane) reactor. Carbon monoxide conversions decreased from 93% to 66% and hydrogen recovery from 90% to 85% at a residence time of 5 s when the Pd was replaced with Pd80w%Cu, due to the lower permeance of the Pd80wt%Cu alloy. SEM-EDS analysis of the membrane tubes suggested that the water-gas shift environment caused pinhole formation in the retentate surfaces of the Pd and Pd80wt%Cu after approximately 8 days of operation. (c) 2007 Elsevier B.V. All rights reserved.