Thin Pd membranes were in situ deposited on macroporous stainless steel (MPSS) tubes using an improved electroless plating method consisting of material filling in the substrate pores, Pd plating on the filled substrate, and recovery and activation of the substrate pores. The Pd/MPSS composite membranes resulted from the filling materials of both aluminum hydroxide gel and Pd/aluminum hydroxide gel have been studied in detail and compared with each other. The hydrogen permeation mechanism through both membranes may be controlled by surface reactions, while the hydrogen permeation flux and activation energy for the membrane resulted from Pd/aluminum hydroxide gel are higher than these for the membrane resulted from aluminum hydroxide gel. In the case of the former membrane, which is almost pinhole free, the hydrogen permeation flux is as high as 0.302 mol/(m(2) s) with a pressure difference of 100 kPa at 773 K. Good membrane stability is also proven by the unchanged membrane surface morphology, the steady hydrogen permeance, and the complete hydrogen selectivity. The deposition mechanism of the membrane has been proposed and interpreted in detail. (c) 2005 Elsevier B.V. All rights reserved.