It is generally regarded that the permeation of hydrogen through palladium or palladium alloy membranes follows the "solution-diffusion" mechanism, which can be described by Sieverts' Law. The hydrogen permeability of actual membrane modules usually differ from those predicted from Sieverts' Law due to the influence of substrate and some other factors. An efficiency (or efficiency factor) has been introduced by many researchers to denote the difference between the actual permeability and those predicted from Sieverts' Law. Extensive experiments were carried out in an electrically heated vessel to study the hydrogen permeation flux and permeation efficiency of Pd-Ag membrane modules with porous stainless steel substrates. The influence of operation conditions on the membrane permeation flux and efficiency was examined. It was observed that the hydrogen permeation flux through the module increased by increasing the temperature and hydrogen pressure in the vessel, while the permeation efficiency increased by increasing the hydrogen pressure in the vessel side and decreasing the membrane temperature. The permeation efficiency was correlated to the vessel temperature (T) and hydrogen pressure difference (P-H(0.5) - P-L(0.5)) for the conditions studied. (c) 2012 Elsevier B.V. All rights reserved.