The H-2 permeance of composite palladium-porous stainless steel (Pd-PSS) membranes was determined: (1) by assuming Sieverts' law (n = 0.5) and (2) by performing a non-linear fit in order to obtain the hydrogen permeance and the n-value. For all membranes (thickness > 15 mu m) the n-value was higher than 0.6 at low temperatures (< 350 degrees C) and close to 0.5 at higher temperatures (> 400 degrees C). The activation of the membrane with the surface either seeded withpalladium or oxidized in air at 350 degrees C for 48 h led to lower n-values indicating that the surface reaction rate even in thick membranes with selectivities (H-2/He) above 400 might still contribute, though to a minor extent, to the overall hydrogen permeation mechanism. For leaky membranes (selectivity << 400) the Knudsen diffusion and viscous flow of molecular H2 through the defects led to n-values as high as 0.75 at 500 degrees C. n-Values higher than 0.5 were also found for Pd-PSS membranes when the PSS support had a large resistance. (c) 2006 Elsevier B.V. All rights reserved.