Nowadays, membrane processes are together with cryogeny, absorption and adsorption considered to be a key technology for gas separation applications. Process selectivity is of primary importance in order to respect the specifications but can in some cases be limited due to a too low membrane selectivity. Parallel to improvements in the membrane selectivity of certain materials, unsteady membrane operations can also be attempted in order to achieve a higher separation selectivity, but this strategy remains largely unexplored. A novel, simple, unsteady-state process, complementary to the strategies already proposed by the previous authors, is reported in this study. A systematic comparison of steady state, short-class (i.e. with time lag) and long-class (i.e. pseudo steady-state) processes is described for O-2/N-2 (PPO membrane), He/Kr (PEMA membrane) and H-2/CO2 (polyimide and a reverse selective PEBAX membrane) separations. Based on a selectivity composition path chart, the maximal purity or process selectivity achievable by steady and unsteady processes was compared. The new long-class process was shown to offer similar or increased selectivity compared to steady-state operation, at the expense of a lower productivity. Finally, the applicability to industrial separations and the remaining challenges are also discussed. (C) 2018 Elsevier Ltd. All rights reserved.