Hydrogen separation from nitrogen containing gas mixture using Pd-Ag membrane shows a time lag and low hydrogen recovery during start-up period. In this study, forced unsteady state operation using a composition modulation of the feed gas was employed as a new operation method to increase the hydrogen recovery and to shorten the time lag. In this work, the performance of Pd75-Ag25 membrane during the separation of hydrogen from H-2/N-2 mixture under forced unsteady state operation at 623 K was investigated. During natural start-up without forced operation, the hydrogen recovery results in 35% and 14% for H-2/N-2 feed gas compositions of 70%/30% and 50%/50%, respectively, while the time lag in the natural operation results in 195 and 147 min for H-2/N-2 feed gas compositions of 70%/ 30% and 50%/50%, respectively. The experimental results show that forced unsteady state operation affects considerably on hydrogen recovery from H-2/N-2 mixture and the time lag during start-up. The maximum hydrogen recovery in the forced unsteady state operation is 60%, obtained at switching time 5 min and H-2/N-2 feed gas composition of 70%/30%, while for H-2/N-2 feed gas composition of 50%/50% the hydrogen recovery is not significantly influenced by switching time. The time lag indicates a profound decay trend monotonically by decreasing the switching time for both H-2/N-2 feed gas compositions. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.