This study reports the poisoning effect of co-existing nitrogen on hydrogen permeation through Pd composite membranes, consisting of thin Pd layers supported on alpha-Al2O3 hollow fibers. Hydrogen permeation of the composite membranes was measured at temperatures of 623-773 K for pure gas permeation in sequence of hydrogen, nitrogen and hydrogen and for mixture gas separation of equimolar H-2/N-2. The composite membranes were defect-free and gave high hydrogen permeance of 31.2m(3)/m(2) h bar at 773 K. However, when the H-2 activated composite membranes were exposed to nitrogen for a certain time, the followed hydrogen permeance decreased in comparison with the original value. The degree of decrease increased with decreasing temperature and with exposure time. Furthermore, when the composite membranes were exposed to the mixture feed of equimolar H2/N2 with certain total flow rate at temperatures of 673-723 K, the hydrogen flux on the permeate side kept stable only for several 100 min then decreased gradually to some extent. And significant reduction could be obtained during the followed pure hydrogen permeance test in comparison with the original value for the fresh membranes. Fortunately, no additional defects were formed on the Pd layers during these processes and the deactivations of the composite membranes were reversible. A certain-time hydrogen treatment at 773 K was sufficient to regenerate the deactivated membranes. The blocking of the active sites on Pd surface for hydrogen diffusion by the formed nitrogen-containing species (NHx, x = 0-2) was responsible for the deactivation of the membranes in the suggested deactivation mechanism. (c) 2006 Elsevier B.V. All rights reserved.