Thin dense Pd/Al2O3 composite membranes were prepared by a vacuum electroless plating technique. By the influence of vacuum effect on the both sides of tubular substrate, thin dense Pd composite membranes with finer and more uniform microstructure were rapidly deposited. These features would significantly improve hydrogen permeation performance and enhance stability as compared with the conventional electroless plating method. Hydrogen permeance of these membranes up to 22.4 m(3)/m(2) It bar and the ideal permselectivity over 3000 were obtained at 500 degrees C. These membranes were stable over a period of 470 h and over 10 temperature cycles and gas-exchanging cycles over the temperature range of 350-480 degrees C under H-2 or Ar atmosphere without any significant changes for hydrogen permeation performance. Moreover, the chemical stability of these membranes in the various H-2-X (X = Ar, N-2, steam, CO2, CH4 and CO) mixtures were investigated. These membranes exhibited good stability over a period of 2000 h and could resist temperature fluctuations under the operating conditions. (C) 2008 Elsevier B.V. All rights reserved.