The long-term stability of Pd-23%Ag/stainless steel composite membranes has been examined in H-2/N-2 mixtures as a function of both temperature and feed pressure. During continuous operation, the membrane shows a good stability at 400 degrees C while the N-2 leakage increases very slowly at a temperature of 450 degrees C (P-feed = 10 bar). After 100 days of operation (P-feed = 5-20 bar, T= 350-450 degrees C), the N-2 permeance equals 7.0 x 10(-9) mol m(-2) s(-1) Pa-1, which indicates that the H-2/N-2 permselectivity still lies around 500, based on a H-2 permeance equal to 3.0 x 10(-6) mol m(-2) s(-1) Pa-1. Despite the generation of small pinholes, a membrane life-time of several (2-3) years (T <= 425 degrees C) is estimated for the experimental conditions employed based on long-term stability tests over 100 days. Post-process characterisation shows a considerable grain growth and micro-strain relaxation in the Pd-23%Ag membrane after the prolonged permeation experiment. Changes in surface area are relatively small. In addition, segregation of Ag to the membrane surfaces is observed. The formation of pinholes is identified as the main source for the increased N-2 leakage during testing at higher temperature. (C) 2008 Elsevier B.V. All rights reserved.