Electroless plating is the technically most facile and most frequently studied method for preparation of PdAg/ceramic composite membranes. Limited high-temperature stability of such membranes requires alloying of sequentially deposited Pd and Ag layers far below their melting points, however. Here it is demonstrated that 600-800 h are needed for forming 2-4 mu m thick, homogeneous alloy layers from Pd Ag bilayers at 823 K under atmospheric H-2 pressure. This is also the time scale on which the activation energy for H-2 permeation becomes stable so that this characteristic can be employed for non-destructive, in-process monitoring of the alloying progress. High-temperature H-2 permeation rates are shown to be less well suited for this purpose because they are not sufficiently sensitive to the homogeneity of PdAg membranes. The activation energies for the well-alloyed membranes indicate that diffusion through the bulk of the PdAg layer limits H-2 permeation through these composite membranes. It is further shown that a fully alloyed Pd75Ag25 membrane tolerates temperature cycling under H-2 well down to 373 K while H-2/N-2 exchanges at that temperature trigger a rapid growth of the N-2 leak rate of that membrane. The defect formation is attributed to mechanical stress caused by the substantial expansion and shrinking of the alloy lattice during hydriding and dehydriding at low temperatures. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.